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 org.omg.CORBA.INTERNAL;

import java.util.*;
import java.io.*;
import static java.lang.Math.*;
public class Reader_ {
    static class Reader {
        final private int BUFFER_SIZE = 1 << 16;
        private DataInputStream din;
        private byte[] buffer;
        private int bufferPointer, bytesRead;

        public Reader() {
            din = new DataInputStream(System.in);
            buffer = new byte[BUFFER_SIZE];
            bufferPointer = bytesRead = 0;
        }

        public Reader(String file_name) throws IOException {
            din = new DataInputStream(new FileInputStream(file_name));
            buffer = new byte[BUFFER_SIZE];
            bufferPointer = bytesRead = 0;
        }

        public String nextLine() throws IOException {
            byte[] buf = new byte[64]; // line length
            int cnt = 0, c;
            while ((c = read()) != -1) {
                if (c == '\n')
                    break;
                buf[cnt++] = (byte) c;
            }
            return new String(buf, 0, cnt);
        }

        public int ni() throws IOException {
            int ret = 0;
            byte c = read();
            while (c <= ' ')
                c = read();
            boolean neg = (c == '-');
            if (neg)
                c = read();
            do {
                ret = ret * 10 + c - '0';
            } while ((c = read()) >= '0' && c <= '9');

            if (neg)
                return -ret;
            return ret;
        }

        public long nl() throws IOException {
            long ret = 0;
            byte c = read();
            while (c <= ' ')
                c = read();
            boolean neg = (c == '-');
            if (neg)
                c = read();
            do {
                ret = ret * 10 + c - '0';
            }
            while ((c = read()) >= '0' && c <= '9');
            if (neg)
                return -ret;
            return ret;
        }

        public double nd() throws IOException {
            double ret = 0, div = 1;
            byte c = read();
            while (c <= ' ')
                c = read();
            boolean neg = (c == '-');
            if (neg)
                c = read();

            do {
                ret = ret * 10 + c - '0';
            }
            while ((c = read()) >= '0' && c <= '9');

            if (c == '.') {
                while ((c = read()) >= '0' && c <= '9') {
                    ret += (c - '0') / (div *= 10);
                }
            }

            if (neg)
                return -ret;
            return ret;
        }

        private void fillBuffer() throws IOException {
            bytesRead = din.read(buffer, bufferPointer = 0, BUFFER_SIZE);
            if (bytesRead == -1)
                buffer[0] = -1;
        }

        private byte read() throws IOException {
            if (bufferPointer == bytesRead)
                fillBuffer();
            return buffer[bufferPointer++];
        }

        public void close() throws IOException {
            if (din == null)
                return;
            din.close();
        }
    }
    public static void main(String[] args) throws IOException {
        new Thread(null, null, "BaZ", 1 << 25) {
            public void run() {
                try {
                    Freak();
                } catch (Exception e) {
                    e.printStackTrace();
                    System.exit(1);
                }
            }
        }.start();
    }
    static long modPow(long x, long p, long m) {
        if (p == 0)
            return 1;
        if (p % 2 == 0)
            return modPow((x * x) % m, p >> 1, m) % m;
        return x * (modPow(x, p - 1, m) % m) % m;
    }
    static long mul(long a, long b, long m) {
        a %= m;
        b %= m;
        return (a * b) % m;
    }
    static long gcd(long a,long b) {
        if(b%a==0)
            return a;
        return gcd(b%a,a);
    }
    public static class Pair implements Comparable<Pair> {
        int x, y;
        Pair(int x, int y) {
            this.x = x;
            this.y = y;
        }

        public int compareTo(Pair o) {
            if (x != o.x)
                return x - o.x;
            return y - o.y;
        }

        public String toString() {
            return x + " " + y;
        }
    }

    //global variable
    static Reader sc = new Reader();
    static StringBuilder sb = new StringBuilder();
    static PrintWriter pw = new PrintWriter(System.out, true);
    static int dir1[][]= {{1,0},{-1,0},{0,1},{0,-1}};
    static int dir2[][] ={{1,-1},{-1,1},{1,1},{-1,-1}};
    static void Freak() throws IOException {
        int n = sc.ni();
        int arr[]= new int[n];
        for(int i =0;i<n;i++)
            arr[i]= sc.ni();

        long ans =0,even=0,odd=0;
        for(int i =0;i<n;i++) {
            ans += arr[i]/2;
            if(arr[i]%2==1) {
//                System.out.println(arr[i]);
                if((i+1)%2==0)
                    even++;
                else
                    odd++;
            }
        }
//        System.out.println("ans"+ans);
//        System.out.println("even"+odd +" "+even);
        ans += min(even,odd);
        sb.append(ans);
        pw.println(sb);
        pw.flush();
        pw.close();
    }
}
