CF 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 × 1squares. 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

给你一个M*N的矩形，问你最多能放多少块2*1的多米诺骨牌。规定多米诺骨牌可以旋转。

简单题，直接贴代码：

[cpp]  view plain  copy

1. #include<cstdio>  
2. #include<cstring>  
3. using namespace std;  
4. int main()  
5. {  
6.     int n,m,i,j;  
7.     scanf("%d%d",&n,&m);  
8.     int x = n/2;  
9.     int y = m/2;  
10.     int ans = x*m;  
11.     if(x*2<n)ans += y;  
12.     printf("%d\n",ans);  
13.     return 0;  
14. }  

贪心的做法，首先使得一边尽可能的放满，剩下的用1*2的多米诺补满。