A. Domino piling

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

#include <iostream>
using namespace std;
int main(){
    int N,M;       //表示N行M列
    int sum=0,a=0;
    ios::sync_with_stdio(false);    //详情见hdoj1754
    while(cin>>N>>M){
        a=N/2;  //一列里可以放的塔罗牌数
        sum=a*M;   // 乘上总列数
        if(N&1) sum+=M/2;    //x&1为真的话，说明x是奇数，二进制，奇数行
        cout<<sum<<endl;     //加上这一行中牌的数量
    }
return 0;
}