CF50A.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.

你有一个 $M \times N$ 的矩形棋盘。同时，你拥有无限数量的标准多米诺骨牌，每块大小为 $2 \times 1$。你可以任意旋转这些骨牌。要求在棋盘上尽可能多地放置多米诺骨牌，且满足以下条件：

1. 每块多米诺骨牌恰好完全覆盖两个方格；
2. 任意两块多米诺骨牌不能重叠；
3. 每块多米诺骨牌必须完全位于棋盘内部（允许其边与棋盘边缘重合）。

求在此限制下最多可放置的多米诺骨牌数量。

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

单行输入中给出两个整数 $M$ 和 $N$ —— 棋盘的尺寸（单位：格子），满足 $1 \leq M \leq N \leq 16$。

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

输出一个数字——最多可以放置的多米诺骨牌数量。