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.io.*;
import java.util.*;
import java.math.*;

public class p1269D {

	public void realMain() throws Exception {

		BufferedReader fin = new BufferedReader(new InputStreamReader(System.in), 1000000);

		String in = fin.readLine();

		String[] ar = in.split(" ");

		int n = Integer.parseInt(ar[0]);
		
		long[] a = new long[n];

		for(int i = 0; i < n; i++) {
			long ret = 0;
			boolean dig = false;
			for (int ch = 0; (ch = fin.read()) != -1; ) {
        			if (ch >= '0' && ch <= '9') {
            				dig = true;
           				ret = ret * 10 + ch - '0';
        			} else if (dig) break;
    			}

			a[i] = ret;
			
		}

		boolean overflow = false;

		long ans = 0;

		long numeven = 0;
		long numodd = 0;

		for(int i = 0; i < n; i++) {
			/*long count = a[i];
			if(overflow) {
				count--;
			}

			ans += count / 2;

			if(count % 2 == 1) {
				overflow = true;
				ans++;
			} else {
				overflow = false;
			}
			System.err.println("count: " + count);
			System.err.println("newans: " + ans);
			*/

			numeven += a[i] / 2;
			numodd += a[i] / 2;

			if(a[i] % 2 == 1) {
				if(i % 2 == 0) {
					numeven++;
				} else {
					numodd++;
				}
			}
			
		}

		//if(overflow) {
		//	ans--;
		//}

		ans = Math.min(numeven, numodd);

		System.out.println(ans);
				



	}


	public static void main(String[] args) throws Exception {
		p1269D a = new p1269D();
		a.realMain();
	}
}