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.*;
public class Main {
	public static void main(String[] args) throws IOException 
	{ 
		FastScanner f = new FastScanner(); 
		int ttt=1;
//		ttt=f.nextInt();
		PrintWriter out=new PrintWriter(System.out);
		outer:for(int tt=0;tt<ttt;tt++) {
			int n=f.nextInt();
		    long cnt = 0;
		    int[] a=new int[n];
		    int[] b=new int[2];
		    for(int i = 0; i < n; i++) {
		        a[i]=f.nextInt();
		        cnt += a[i];
		        if(a[i] % 2 == 1) {
		            b[i % 2]++;
		        }
		    }
		    cnt -= Math.abs(b[0] - b[1]);
		    System.out.println(cnt/2);
		}
		out.close();
	} 
	static void sort(int[] p) {
        ArrayList<Integer> q = new ArrayList<>();
        for (int i: p) q.add( i);
        Collections.sort(q);
        for (int i = 0; i < p.length; i++) p[i] = q.get(i);
    }
	static class FastScanner {
		BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
		StringTokenizer st=new StringTokenizer("");
		String next() {
			while (!st.hasMoreTokens())
				try {
					st=new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			return st.nextToken();
		}
		
		int nextInt() {
			return Integer.parseInt(next());
		}
		int[] readArray(int n) {
			int[] a=new int[n];
			for (int i=0; i<n; i++) a[i]=nextInt();
			return a;
		}
		long nextLong() {
			return Long.parseLong(next());
		}
		double nextDouble() {
			return Double.parseDouble(next());
		}
		long[] readLongArray(int n) {
			long[] a=new long[n];
			for (int i=0; i<n; i++) a[i]=nextLong();
			return a;
		}
	}
} 	