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

 
public class Main{
	/*
	 * use Integer for sorting
	 * avoid object comparison
	 * NEVER APPEND A STRING -> APPEND char by char
	 * 
	 */
	public static class FastReader {
		BufferedReader br;
		StringTokenizer root;
		
 
		public FastReader() {
			br = new BufferedReader(new InputStreamReader(System.in));
		}
 
		String next() {
			while (root == null || !root.hasMoreTokens()) {
				try {
					root = new StringTokenizer(br.readLine());
				} catch (Exception r) {
					r.printStackTrace();
				}
			}
			return root.nextToken();
		}
 
		int nextInt() {
			return Integer.parseInt(next());
		}
 
		double nextDouble() {
			return Double.parseDouble(next());
		}
 
		long nextLong() {
			return Long.parseLong(next());
		}
 
		String nextLine() {
			String str = "";
			try {
				str = br.readLine();
			} catch (Exception r) {
				r.printStackTrace();
			}
			return str;
		}
	}
	
	public static PrintWriter out = new PrintWriter (new BufferedOutputStream(System.out));
	static long mod = (long) (1e9+7);
	static long cf = 998244353;
    static final long MAX = (long) 1e8;
    public static List<Integer>[] edges;
    public static int[][] parent;
    public static long[] fac;
    public static int N = 400000+200;
    public static int x = 0;
    public static boolean[] visited;
	public static void main(String[] args) {
		FastReader sc = new FastReader();
		
		int n = sc.nextInt();
		int[] a = new int[n];
		for(int i=0;i<n;++i) a[i] = sc.nextInt();
		long b=0,w=0;
		for(int i=0;i<n;++i) {
			if((i&1) == 1) {
				b+=(a[i]+1)/2;
				w+=a[i]/2;
			}else {
				b+=a[i]/2;
				w+=(a[i]+1)/2;
			}
		}
		out.print(Math.min(w, b));
		out.close();
	}
}