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.DataInputStream;
import java.io.IOException;
import java.io.PrintWriter;

public class P1268B2 {
    public static void main(String[] args) throws IOException {
        InputReader2 ir = new InputReader2();
        PrintWriter pw = new PrintWriter(System.out);

        int n = ir.nextInt();
        int[] cols = new int[n];
        for (int i = 0; i < n; i++) {
            cols[i] = ir.nextInt();
        }

        long a = 0L;
        long b = 0L;
        for (int i = 0; i < n; i += 2) {
            int tmp = cols[i] / 2;
            a += tmp;
            b += (cols[i] - tmp);
        }
        for (int i = 1; i < n; i += 2) {
            int tmp = cols[i] / 2;
            b += tmp;
            a += (cols[i] - tmp);
        }

        pw.println(Math.min(a, b));
        pw.close();
    }

    private static class InputReader2 {
        private final int BUFFER_SIZE = 1 << 16;
        private final DataInputStream dis;
        private final byte[] buffer;
        private int bufferPointer, bytesRead;

        public InputReader2() {
            dis = new DataInputStream(System.in);
            buffer = new byte[BUFFER_SIZE];
            bufferPointer = bytesRead = 0;
        }

        private int nextInt() throws IOException {
            int ret = 0;
            byte c = read();
            while (c <= ' ')
                c = read();
            do {
                ret = ret * 10 + c - '0';
            } while ((c = read()) >= '0' && c <= '9');
            return ret;
        }

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

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