Valera loves his garden, where n fruit trees grow.

This year he will enjoy a great harvest! On the i-th tree bi fruit grow, they will ripen on a day number ai. Unfortunately, the fruit on the tree get withered, so they can only be collected on day ai and day ai + 1 (all fruits that are not collected in these two days, become unfit to eat).

Valera is not very fast, but there are some positive points. Valera is ready to work every day. In one day, Valera can collect no more than v fruits. The fruits may be either from the same tree, or from different ones. What is the maximum amount of fruit Valera can collect for all time, if he operates optimally well?

Input

The first line contains two space-separated integers n and v (1 ≤ n, v ≤ 3000) — the number of fruit trees in the garden and the number of fruits that Valera can collect in a day. 

Next n lines contain the description of trees in the garden. The i-th line contains two space-separated integers ai and bi (1 ≤ ai, bi ≤ 3000) — the day the fruits ripen on the i-th tree and the number of fruits on the i-th tree.

Output

Print a single integer — the maximum number of fruit that Valera can collect. 

Examples

Input

2 3
1 5
2 3


Output

8


Input

5 10
3 20
2 20
1 20
4 20
5 20


Output

60

Note

In the first sample, in order to obtain the optimal answer, you should act as follows. 

  * On the first day collect 3 fruits from the 1-st tree. 
  * On the second day collect 1 fruit from the 2-nd tree and 2 fruits from the 1-st tree. 
  * On the third day collect the remaining fruits from the 2-nd tree. 



In the second sample, you can only collect 60 fruits, the remaining fruit will simply wither.


        import java.io.*;


         
            import java.math.BigInteger;
        import java.text.DecimalFormat;
        import java.util.*;

        
        
             public class Main {
            	public static InputReader in;
                public static PrintWriter pw;
             
             
             
            	public static void main(String args[]) 
            	{
            		new Thread(null, new Runnable() {
                        public void run() {
                            try{
                                solve();
                            }
                            catch(Exception e){
                                e.printStackTrace();
                            }
                        }
                    }, "1", 1 << 26).start();
                }
            	static ArrayList<Integer> g[];
            	static ArrayList<Integer> h[];
            	static boolean visited[];
            	static int[] parent;
            	static int par[];
            	static int degree[];
            	static int edjes=0;
            	static int start=-1;
            	static int end=-1;
            	static int Total=0;
            	static int time1[];
            	static int time2[];
            	static int glob=0;
            	static int[] ans;
            	static boolean vis2[][];
            	//static long sum1=0;
            	//static long val[];
                static ArrayList<Integer> levels[];
                static int max=0;
                static int lev=1;
                static ArrayList<Integer> nodes[];
                static ArrayList<Integer> values[];
                static int depth[];
                static boolean found=false;
               // static long sm[];
                static int sum1=0;
                static int pre[][];
                static int subtree[];
               static  int cnt=0;
               static  HashMap<String,Integer> hm;
               static int sm[];
               static int size[];
               static long mul[];
               static long d[]=new long[1000000];
               static int tot=0;
               static long highest=(long)1e9;
               static boolean bit=false;
                static Stack<Integer> tt=new Stack();
               static HashSet<Character> set;
         	   static long fact[],num[];
         	   static long MOD=1000000007;
         	   static int value[];static int premen[];
         	   static int count=0,index=0;
         	   static double ans11[];
         	   static int price[];
         	   static ArrayList<String> hp[][];
         	   static boolean visit[][];
         	   static int cnu[]=new int[1000001];
         	   static int label=1;
         	   static int his[][];
         	   static int size1=0;
         	   static int up[],down[];
         	   static int TotalUp=0;
         	   static long dist[];
         	//   static HashMap<String,Integer> hm1=new HashMap();
         	  // static HashMap<Integer,Integer> hm2=new HashMap();
         	   static TreeSet<Long> set1=new TreeSet();
         	  static  long DP[][][]=new long[20][200][1500];
         	 	static int t=-1,n=-1;
         	 	static int two[],min[],r[];
         	 	static int total[];static int black[];
         	 	static int Parent[];
         	 	static int max2[];
         	// 	static int tin[],tout[],a[];
         	 public static void solve() throws FileNotFoundException{
            		
            	
            		 in = new InputReader(System.in);
            		 pw = new PrintWriter(System.out);  
            	/*	 String p=in.readString();
            	     char c[]=p.toCharArray();
            	     int z[]=new int[p.length()];
            	     z[0]=0;
            	     int left=0;
            	     int right=0;
            	     for(int i=1;i<p.length();i++)
            	     {
            	    	 if(i>right)
            	    	 {
            	    		 
            	    		 left=right=i;
            	    		 while(right<p.length()&&c[right]==c[right-left])
            	    			 right++;
            	    		 z[i]=right-left;
            	    		 right--;
            	    	 }
            	    	 else
            	    	 {
            	    		 int k=i-left;
            	    		 if(z[k]<right-i+1)
            	    		 {
            	    			 z[i]=z[k];
            	    		 }
            	    		 else
            	    		 {
            	    			 left=i;
            	    			 while(right<p.length()&&c[right]==c[right-left])
                	    			 right++;
                	    		 z[i]=right-left;
                	    		 right--;
            	    		 }
            	    	 }
            	    	  
            	     }*/
            		 int n=in.nextInt();
            		 int v=in.nextInt();
            	        int days[]=new int[5678];
            	        int max=0;
            	        for(int i=0;i<n;i++){
            	        	int p2=in.nextInt();
            	        	int q2=in.nextInt();
            	        	days[p2]+=q2;
            	        	max=Math.max(max,p2);
            	        }
            			int curr=0;
            			int ans=0;
            			for(int i=1;i<=(max+1);i++)
            			{
            					if(days[i]<=v)
            					{
            						ans+=days[i];
            					}
            					else if(days[i]>v)
            					{
            						ans+=v;
            						if(days[i]-v<=v)
            						days[i+1]+=(days[i]-v);
            						else
            							days[i+1]+=v;
            					}
            				
            			}
            			System.out.println(ans);
         	 }
         	 public static boolean check(int a[][],int b[][],int n,int m)
         	 {
         		for(int k=0;k<n;k++)
    			{
    				for(int l=0;l<m;l++)
    				{
    					if(a[k][l]!=b[k][l])
    					{
    						return false;
    						//break;
    					}
    				}
    			}
         		return true;
         	 }
        
            
          		public static void bfs(int x)
         	 {
         		 visited[x]=true;
         		 Queue<Integer> Q=new LinkedList<Integer>();
         		 Q.add(x);
         		 while(!Q.isEmpty())
         		 {
         			 int count=Q.size();
         			 while(count>0)
         			 {
         				 int p=Q.poll();
         				 visited[p]=true;
         				 for(int y:g[p])
         				 {
         					 if(!visited[y])
         					 {
         						 Q.add(y);
         					 }
         				 }
         				 count--;
         			 }
         		 }
         	 }
         	 public static void depth(int curr,int parent)
         	 {
         		 visited[curr]=true;
         		 for(int x:g[curr])
         		 {
         			 if(!visited[x])
         			 {
         				 depth[x]=depth[curr]+1;
         				 depth(x,curr);
         			 }
         		 }
         	 }
         	
         	 static int Par[]=new int[202];
         	 public static void Height(int cur,int par){
         		 Par[cur]=par;
         		 visited[cur]=true;
         		 int t=0;
         		 for(int x:g[cur])
         		 {
         			 if(!visited[x])
         			 { 
         				 t++;
         				 Height(x,cur);
         				 high[cur]=Math.max(high[cur], high[x]+1);
         			 }
         		 }
         		 if(t==0)
         		 {
         			 high[cur]=0;
         		 }
         	
         	 }
         	 static int dia[]=new int[202];
         	 static int high[]=new int[292];
         	 public static void Diameter(int curr,int parent)
         	 {
         		visited[curr]=true;
         		int trees=0;
         		ArrayList<Integer> set=new ArrayList();
         	
         		for(int x:g[curr])
         		{
         			if(!visited[x])
         			{
         				trees++;
         				Diameter(x,curr);
         				dia[curr]=Math.max(dia[curr],high[x]);
         				set.add(high[x]);
         			}
         		}
         		if(set.size()>=2)
         		{
         			Collections.sort(set,Collections.reverseOrder());
         			dia[curr]=Math.max(dia[curr],set.get(0)+set.get(1)+2);
         		}
         		else if(set.size()>0)
         		{
         			dia[curr]=Math.max(dia[curr],set.get(0)+1);
         		}
         		if(trees==0)
         		{
         			dia[curr]=0;
         		}
         
         	 }
         	 
              	 
         	 public static boolean IsPal(String s)
         	 {
         		 char c[]=s.toCharArray();
         		 for(int i=0;i<c.length/2;i++)
         		 {
         			 if(c[i]!=c[s.length()-1-i])
         				 return false;
         		 }
         		 return true;
         	 }
         	public static void Merge(long[] a,int p,int r){
                if(p<r){
                    int q = (p+r)/2;
                    Merge(a,p,q);
                    Merge(a,q+1,r);
                    Merge_Array(a,p,q,r);
                }
            }
            public static void Merge_Array(long a[],int p,int q,int r){
               long b[] = new long[q-p+1];
                long c[] = new long[r-q];
                for(int i=0;i<b.length;i++)
                    b[i] = a[p+i];
                for(int i=0;i<c.length;i++)
                    c[i] = a[q+i+1];
                int i = 0,j = 0;
                for(int k=p;k<=r;k++){
                    if(i==b.length){
                        a[k] = c[j];
                        j++;
                    }
                    else if(j==c.length){
                        a[k] = b[i];
                        i++;
                    }
                    else if(b[i]<c[j]){
                        a[k] = b[i];
                        i++;
                    }
                    else{
                        a[k] = c[j];
                        j++;
                    }
                }
            }
         	 
         	 
         	 
         	
         	
               
         	  public static void dfs1(int curr,int parent)
         	  {
         		  visited[curr]=true;
         		  
         		  for(int x:g[curr])
         		  {
         			  
         			  if(x==parent)continue;
         			  depth[x]=depth[curr]+1;
         			  up[x]=up[curr];
         			  if(h[x].contains(curr)){
         				  up[x]++;
         				  TotalUp++;
         			  }
         			  dfs1(x,curr);
         		  }
         	  }
         	 
                
                
                public static void dfs(int curr,int Parent)
                {
                	visited[curr]=true;
                	size[curr]=1;
                	parent[curr]=Parent;
                	count++;
                	if(value[curr]>0)
                	{
                		index=value[curr];
                	}
                	for(int x:g[curr])
                	{
                		if(!visited[x])
                		{
                			dfs(x,curr);
                			size[curr]+=size[x];
                		}
                	}
                }
                static long ncr(long a, long b){
        			if(b>a || b<0) return 0;
        			long f = (fact[(int) a]*modInverse(((fact[(int) b]*fact[(int) (a-b)])%MOD), MOD))%MOD;
        			return f;
        		}
        		static long modInverse(long a, long mOD2){
                    return  power(a, mOD2-2, mOD2);
        }
        static long power(long x, long y, long m)
        {
            if (y == 0)
                return 1;
            long p = power(x, y/2, m) % m;
            p = (p * p) % m;
         
            return (y%2 == 0)? p : (x * p) % m;
        }
         
                public static long gcd(long x, long y) {
            		if (x == 0)
            			return y;
            		else
            			return gcd( y % x,x);
            	}	
                public static long power(long a,long b)
        		{
        			long result=1;
        			while(b>0)
        			{
        				if(b%2==1)
        					result*=a;
        				a=a*a;
        				b/=2;
        			}
        			return result;
        		}
                static class Pair implements Comparable<Pair>{
                	    int l;int r;
                		Pair(int val,int index){
                	     	this.l=val;
                			this.r=index;
                			
                		}
                		@Override
                		public int compareTo(Pair o) {
                                if(l!=o.l)
                                	return l-o.l;
                                else
                                	return r-o.r;
                		}
                				
                	}
            	static class InputReader {
             
            		private InputStream stream;
            		private byte[] buf = new byte[8192];
            		private int curChar, snumChars;
            		private SpaceCharFilter filter;
             
            		public InputReader(InputStream stream) {
            			this.stream = stream;
            		}
             
            		public int snext() {
            			if (snumChars == -1)
            				throw new InputMismatchException();
            			if (curChar >= snumChars) {
            				curChar = 0;
            				try {
            					snumChars = stream.read(buf);
            				} catch (IOException e) {
            					throw new InputMismatchException();
            				}
            				if (snumChars <= 0)
            					return -1;
            			}
            			return buf[curChar++];
            		}
             
            		public   int nextInt() {
            			int c = snext();
            			while (isSpaceChar(c))
            				c = snext();
            			int sgn = 1;
            			if (c == '-') {
            				sgn = -1;
            				c = snext();
            			}
            			int res = 0;
            			do {
            				if (c < '0' || c > '9')
            					throw new InputMismatchException();
            				res *= 10;
            				res += c - '0';
            				c = snext();
            			} while (!isSpaceChar(c));
            			return res * sgn;
            		}
             
            		public long nextLong() {
            			int c = snext();
            			while (isSpaceChar(c))
            				c = snext();
            			int sgn = 1;
            			if (c == '-') {
            				sgn = -1;
            				c = snext();
            			}
            			long res = 0;
            			do {
            				if (c < '0' || c > '9')
            					throw new InputMismatchException();
            				res *= 10;
            				res += c - '0';
            				c = snext();
            			} while (!isSpaceChar(c));
            			return res * sgn;
            		}
             
            		public int[] nextIntArray(int n) {
            			int a[] = new int[n];
            			for (int i = 0; i < n; i++)
            				a[i] = nextInt();
            			return a;
            		}
             
            		public String readString() {
            			int c = snext();
            			while (isSpaceChar(c))
            				c = snext();
            			StringBuilder res = new StringBuilder();
            			do {
            				res.appendCodePoint(c);
            				c = snext();
            			} while (!isSpaceChar(c));
            			return res.toString();
            		}
             
            		public boolean isSpaceChar(int c) {
            			if (filter != null)
            				return filter.isSpaceChar(c);
            			return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
            		}
             
            		public interface SpaceCharFilter {
            			public boolean isSpaceChar(int ch);
            		}
            	}
            }
          
     

