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>
import java.util.*;

import java.util.*;

public class ArrayList {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
           int n=sc.nextInt();
           long s=0,t=0;
           for(int i=0;i<n;i++) {
               int a = sc.nextInt();
               if (a % 2 == 1)
                   t += 2 * (i % 2) - 1;
               s += a;
           }
               if(t < 0) t = -t;
               System.out.println((s-t)/2);
               }

           }