You are given a Young diagram. 

Given diagram is a histogram with n columns of lengths a_1, a_2, …, a_n (a_1 ≥ a_2 ≥ … ≥ a_n ≥ 1).

<image> Young diagram for a=[3,2,2,2,1].

Your goal is to find the largest number of non-overlapping dominos that you can draw inside of this histogram, a domino is a 1 × 2 or 2 × 1 rectangle.

Input

The first line of input contain one integer n (1 ≤ n ≤ 300 000): the number of columns in the given histogram.

The next line of input contains n integers a_1, a_2, …, a_n (1 ≤ a_i ≤ 300 000, a_i ≥ a_{i+1}): the lengths of columns.

Output

Output one integer: the largest number of non-overlapping dominos that you can draw inside of the given Young diagram.

Example

Input


5
3 2 2 2 1


Output


4

Note

Some of the possible solutions for the example:

<image> <image>
//package learning;
import java.util.*;
import java.io.*;
import java.lang.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;
public class NitsLocal {
    static ArrayList<String> s1;
    static boolean[] prime;
    static int n = (int)1e7;
    
     static void sieve() {
Arrays.fill(prime	, true);
prime[0] = prime[1] = false;
for(int i = 2 ; i * i <= n ; ++i) {
if(prime[i]) {
for(int k = i * i; k<= n ; k+=i) {
prime[k] = false;
}
}
}
 
}
    public static void main(String[] args) {
        InputReader sc = new InputReader(System.in);
        n *= 2;
           prime  = new boolean[n + 1];
           //sieve();
                prime[1] = false;
                /*
              int n = sc.ni();     
              int []a = sc.nia(n);
              int mod = 1000000007;
              long ans = 0;
              int f = 0;
              int q = sc.ni();
              while(q-- > 0)
              {
                  int num = sc.ni();
                  ans = 0;
                  if(num == 0)
                  {
                      a[sc.ni()-1] = sc.ni();
                  }
                 else
                  {
                      int l = sc.ni();
                      int r = sc.ni();
                    for(int i=l-1;i<r;i++)
                   {
                  
                   for(int j=i+1;j<r;j++)
                   {
                      f = 0;
                   
                      int x = a[i];
                      int y = a[j];
                      while(x > 0 && y > 0)
                      {
                          int y1 = x%10;
                          int y2 = y%10;
                          if(y1 > 0 && y2 > 0)
                          {
                              f = 1;
                          }
                          x /= 10;
                          y /= 10;
                      }
                      
                    
                    
                   
                     if(f==0)
                     {
                         ans = (ans + ((((long)a[i]%mod) * ((long)a[j]%mod)))%mod)%mod;
                     }
                  }
              }
              
              w.println(ans);
                  }
              }
          
            */
                /*
               int t  = sc.ni();
               while(t-- > 0)
               {
                   int n = sc.ni();
                   Student []st = new Student[n];
                   for(int i=0;i<n;i++)
                   {
                       st[i] = new Student(sc.ni(),sc.ni());
                   }
                   Arrays.sort(st,(p,q)->p.id-q.id);
                   if(n==1)
                       w.println("0");
                   else
                   {
                    int prev = 0;
                    int adv = 0;
                    int max = -1;
                    int min = 1000000000;
                    for(int i=0;i<n;i++)
                    {
                        min = Math.min(st[i].y,min);
                        max = Math.max(st[i].y,max);
                    }
                        if(st[0].y > st[1].y)
                        {
                            prev = st[0].id;
                        }
                        else
                            prev= st[0].y;
                         if(st[n-1].id < st[n-2].y)
                        {
                            adv = st[n-1].id;
                        }
                        else
                            prev= st[n-1].y;
                         w.println(adv-prev);
                    
                   }
               }
           */
             
                int n = sc.ni();
                int prev = 0;
                long sum = 0;
                long x = 0;
                long y = 0;
                for(int i=0;i<n;i++)
                {
                    int a = sc.ni();
                   if(i%2==0)
                   {
                       x += (long)a/2;
                       y += (long)(a+1)/2;
                   }
                   else
                   {
                       x += (long) (a+1)/2;
                       y +=  (long) (a/2);
                   }
                }
                w.println(Math.min(x,y));
                    
       w.close();
       
        
    }
   
            
    public static int upperBound(int []arr,int n,int num)
    {
        int l = 0;
        int r = n;
        while(l < r)
        {
            int mid = (l+r)/2;
            if(num >= arr[mid])
            {
                l = mid +1;
            }
            else
                r = mid;
        }
        return l;
    }
    public static int countSetBits(int number){
        int count = 0;
        while(number>0){
            ++count;
            number &= number-1;
        }
        return count;
    }
    
     static  class CustomerSortingComparator 
                                implements Comparator<Temp> {
 
    @Override
    public int compare(Temp cust1, Temp cust2) {
 
        // all comparison
        int compareName = 0;
        if(cust1.a < cust2.a)
            return 1;
        
        else if(cust1.a > cust2.a)
        return -1;
        else
        {
            if(cust1.b < cust2.b)
                return 1;
            else
                return -1;
            
        }
       
        }
    
    }
      

    public static long pow(long i, long j) {
		long res=1L;
		for(int a=0;a<j;a++) res*=i;
		return res;
	}
    public static long powmod(long i, long j,long MOD) {
		if(j==0) return 1L;
		if(j%2==0) {
			long temp=powmod(i,j/2,MOD);
			return (temp*temp)%MOD;
		}
		return (i*powmod(i,j-1,MOD))%MOD;
	}
    static HashSet<Integer> fac;
    public static void primeFactors(int n) 
    { 
        // Print the number of 2s that divide n 
        int t = 0;
        while (n%2==0) 
        { 
            fac.add(2);
            n /= 2; 
        } 
  
        // n must be odd at this point.  So we can 
        // skip one element (Note i = i +2) 
        for (int i = 3; i*i <= n; i+= 2) 
        { 
            // While i divides n, print i and divide n 
            while (n%i == 0) 
            { 
                fac.add(i);
                n /= i; 
            } 
        } 
  
        // This condition is to handle the case whien 
        // n is a prime number greater than 2 
        if (n > 2) 
            fac.add(n);
    } 
     static long modexp(long x,long n,long M)
{
    long power = n;
    long result=1;
    x = x%M;
    while(power>0)
    {
        if(power % 2 ==1)
            result=(result * x)%M;
        x=(x*x)%M;
        power = power/2;
    }
    return result;
}
    static int gcd(int a,int b)
    {
        if(a==0)
            return b;
        else
            return gcd(b%a,a);
    }
    static class Temp{
        int a;
        int b;
        int c;
        int d;
      
        Temp(int a,int b,int c,int d)
        {
            this.a = a;
            this.b = b;
            this.c  =c;
            this.d = d;
            //this.d = d;
        }
    }
    static class Query{
        int p;
        int index;
        int value;
        public Query(int p,int index,int value)
        {
            this.p = p;
            this.index = index;
            this.value = value;
            
        }
    }
    static int nextPowerOf2(int n) 
    { 
        int count = 0; 
  
        // First n in the below  
        // condition is for the  
        // case where n is 0 
        if (n > 0 && (n & (n - 1)) == 0) 
            return n; 
  
        while(n != 0) 
        { 
            n >>= 1; 
            count += 1; 
        } 
  
        return 1 << count; 
    } 
    
   static long sum1(int t1,int t2,int x,int []t)
   {
       int mid = (t2-t1+1)/2;
  if(t1==t2)
      return 0;
    
   else
   return sum1(t1,mid-1,x,t) + sum1(mid,t2,x,t);
    
     
   }
  
   
    static String replace(String s,int a,int n)
    {
        char []c = s.toCharArray();
        for(int i=1;i<n;i+=2)
        {
            int num = (int) (c[i] - 48);
            num += a;
            num%=10;
            c[i] = (char) (num+48);
        }
        return new String(c);
    }
    static String move(String s,int h,int n)
    {
        h%=n;
        char []c = s.toCharArray();
        char []temp = new char[n];
        for(int i=0;i<n;i++)
        {
            temp[(i+h)%n] = c[i];
        }
        return new String(temp);
    }
    
     public static int ip(String s){
return Integer.parseInt(s);
}
     static class multipliers implements Comparator<Long>{
  
         
  public int compare(Long a,Long b) {
   if(a<b)
    return 1;
   else if(b<a)
    return -1;
   else
    return 0;
  }
 }
     static class multipliers1 implements Comparator<Student>{
  
  public int compare(Student a,Student b) {
   if(a.y<b.y)
    return 1;
   else if(b.y<a.y)
    return -1;
   else
   {
       if(a.id < b.id)
           return 1;
       else if(b.id<a.id)
           return -1;
       else
           return 0;
           
    //return 0;
   }
  }
 }
    // Java program to generate power set in 
// lexicographic order. 
 
    
    
    
      static class InputReader {
 
private final InputStream stream;
private final byte[] buf = new byte[8192];
private int curChar, snumChars;
 
public InputReader(InputStream st) {
this.stream = st;
}
 
public int read() {
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 ni() {
int c = read();
while (isSpaceChar(c)) {
c = read();
}
int sgn = 1;
if (c == '-') {
sgn = -1;
c = read();
}
int res = 0;
do {
res *= 10;
res += c - '0';
c = read();
} while (!isSpaceChar(c));
return res * sgn;
}
 
public long nl() {
int c = read();
while (isSpaceChar(c)) {
c = read();
}
int sgn = 1;
if (c == '-') {
sgn = -1;
c = read();
}
long res = 0;
do {
res *= 10;
res += c - '0';
c = read();
} while (!isSpaceChar(c));
return res * sgn;
}
 
public int[] nia(int n) {
int a[] = new int[n];
for (int i = 0; i < n; i++) {
a[i] = ni();
}
return a;
}
 
public String rs() {
int c = read();
while (isSpaceChar(c)) {
c = read();
}
StringBuilder res = new StringBuilder();
do {
res.appendCodePoint(c);
c = read();
} while (!isSpaceChar(c));
return res.toString();
}
 
public String nextLine() {
int c = read();
while (isSpaceChar(c))
c = read();
StringBuilder res = new StringBuilder();
do {
res.appendCodePoint(c);
c = read();
} while (!isEndOfLine(c));
return res.toString();
}
 
public boolean isSpaceChar(int c) {
return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
}
 
private boolean isEndOfLine(int c) {
return c == '\n' || c == '\r' || c == -1;
}
 
}
 
 
 
 
 
   
static PrintWriter w = new PrintWriter(System.out);
                  static class Student
    {
        int id;
        //int x;
       
        int y;
        //long z;
        
        Student(int id,int y)
        {
            this.id = id;
            //this.x = x;
            //this.s = s;
            this.y = y;
           // this.z = z;
            
            
            
           
        }
        
    }
}