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.BufferedReader;
import java.text.DecimalFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.*;
import java.io.*;

public class abc {
	static PrintWriter pw;

	static class FastReader {
		BufferedReader br;
		StringTokenizer st;

		public FastReader() {
			br = new BufferedReader(new InputStreamReader(System.in));
		}

		String next() {
			while (st == null || !st.hasMoreElements()) {
				try {
					st = new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
			return st.nextToken();
		}

		int nextInt() {
			return Integer.parseInt(next());
		}

		long nextLong() {
			return Long.parseLong(next());
		}

		double nextDouble() {
			return Double.parseDouble(next());
		}

		String nextLine() {
			String str = "";
			try {
				str = br.readLine();
			} catch (IOException e) {
				e.printStackTrace();
			}
			return str;
		}

	}

	public static long gcd(long a, long b) {
		return b == 0 ? a : gcd(b, a % b);
	}

	public static boolean isPrime(long n) {
		if (n == 2)
			return true;
		long i = 2;
		while (i * i <= n) {
			if (n % i == 0)
				return false;
			i++;
		}
		return true;
	}

	public static long[] remove(long n) {
		long res[] = new long[1000000000];
		long rese = 0;
		int i = 0;
		while (n > 0) {
			long dig = n % 10;
			n = n / 10;
			if (dig > 0) {
				rese = dig;
				res[i++] = rese;
			}
		}
		return res;
	}

	static int ceil(int x, int y) {
		return (x % y == 0 ? x / y : (x / y + 1));
	}

	static long ceil(long x, long y) {
		return (x % y == 0 ? x / y : (x / y + 1));
	}

	static int max(int x, int y) {
		return Math.max(x, y);
	}

	static int min(int x, int y) {
		return Math.min(x, y);
	}

	static int abs(int x) {
		return Math.abs(x);
	}

	static long abs(long x) {
		return Math.abs(x);
	}

	static int log2(int N) {
		int result = (int) (Math.log(N) / Math.log(2));
		return result;
	}

	static long max(long x, long y) {
		return Math.max(x, y);
	}

	static long min(long x, long y) {
		return Math.min(x, y);
	}

	public static class pair {
		int x;
		int y;

		public pair(int a, int b) {
			x = a;
			y = b;
		}
	}

	public static class Comp implements Comparator<pair> {
		public int compare(pair a, pair b) {
			if (a.x != b.x) {
				return a.x - b.x;
			} else {
				return a.y - b.y;
			}
		}
	}

	public static void extract(ArrayList<Integer> ar, int k, int d) {
		int c = 0;
		for (int i = 1; i < k; i++) {
			int x = 0;
			boolean dm = false;
			while (x > 0) {
				long dig = x % 10;
				x = x / 10;
				if (dig == d) {
					dm = true;
					break;
				}
			}
			if (dm)
				ar.add(i);
		}
	}

	public static void dfs(int index, boolean vis[], int a[], int b[], int n) {
		vis[index] = true;
		for (int i = 0; i < n; i++) {
			if (!vis[i] && (a[i] == a[index] || b[i] == b[index]))
				dfs(i, vis, a, b, n);
		}
	}
// counts the set(1) bit of a number
	public static long countSetBitsUtil(long x) {
		if (x <= 0)
			return 0;
		return (x % 2 == 0 ? 0 : 1) + countSetBitsUtil(x / 2);
	}
	
//tells whether a particular index has which bit of a number	
	public static int getIthBitsUtil(int x , int y)
	{
		return (x&(1<<y))!=0 ? 1 : 0;
	}
	
	public static void swap(long x,long y) {
		x=x^y;
		y=y^x;
		x=x^y;
	}
	
	public static double decimalPlaces(double sum) {
		DecimalFormat df = new DecimalFormat("#.00");
		String angleFormated = df.format(sum);
		double fin = Double.parseDouble(angleFormated);
		return fin;
	}

//use collections.swap for swapping 
	static boolean isSubSequence(String str1, String str2, int m, int n) {
		int j = 0;

		for (int i = 0; i < n && j < m; i++)
			if (str1.charAt(j) == str2.charAt(i))
				j++;

		return (j == m);
	}

	static long[] sum(int k, long n) {
		long a[] = new long[k];
		int c = 0;
		while (n > 0) {
			a[c++] = n % 10;
			n = n / 10;
		}
		return a;
	}

	static long pow(long base, long power) {
		if (power == 0) {
			return 1;
		}
		long result = pow(base, power / 2);
		if (power % 2 == 1) {
			return result * result * base;
		}
		return result * result;
	}

	public static void main(String[] args) throws Exception {

		FastReader sc = new FastReader();
		pw = new PrintWriter(System.out);

		// use arraylist as it use the concept of dynamic table(amortized analysis)
		// Arrays.stream(array).forEach(a -> Arrays.fill(a, 0));
		int t=1;
		while(t-->0)
		{
			int n=sc.nextInt();
			int k=sc.nextInt();
			int a[]=new int[n];
			int b[]=new int[n];
			for(int i=0;i<n;i++)
			{
				a[i]=sc.nextInt();
				b[i]=sc.nextInt();
			}
			int ans=0,f=0;
			for(int d=1;d<=3001;d++)
			{
				int c=0;
				for(int i=0;i<n;i++)
				{
					if(a[i]==d)
						c+=b[i];
				}
					if(c+f <= k)
					{
						ans+=f+c;
						f=0;
					}
					else
					{
						ans+=k;
						int aux=k-f;
						if(aux<0)
							aux=0;
						f=c-aux;
					}
			}
			pw.println(ans);
		}
		pw.flush();
	}
}
