C

Problem description

You are given a rectangular board of M × N squares. Also you are given an unlimited number of standard domino pieces of 2 × 1 squares. You are allowed to rotate the pieces. You are asked to place as many dominoes as possible on the board so as to meet the following conditions:

1. Each domino completely covers two squares.

2. No two dominoes overlap.

3. Each domino lies entirely inside the board. It is allowed to touch the edges of the board.

Find the maximum number of dominoes, which can be placed under these restrictions.

Input

In a single line you are given two integers M and N — board sizes in squares (1 ≤ M ≤ N ≤ 16).

Output

Output one number — the maximal number of dominoes, which can be placed.

Examples

Input

2 4

Output

4

Input

3 3

Output

4
解题思路：题目很简单，要求在一个m*n的矩形中用一张2*1的扑克牌按方格尽量铺满，并且不能超过矩形界限但可以贴着矩形边缘，直接取矩形面积的一半即可，水过！
AC代码：

#include <bits/stdc++.h>
using namespace std;
int main()
{
    int m,n;
    cin>>m>>n;
    cout<<(m*n/2)<<endl;
    return 0;
}