题目描述
There is a right triangle ABC with ∠ABC=90°.
Given the lengths of the three sides, |AB|,|BC| and |CA|, find the area of the right triangle ABC.
It is guaranteed that the area of the triangle ABC is an integer.
Constraints
- $1 \leq |AB|,|BC|,|CA| \leq 100$
- All values in input are integers.
- The area of the triangle ABC is an integer.
Input
Input is given from Standard Input in the following format:
|AB| |BC| |CA|
Output
Print the area of the triangle ABC.
Examples
Input
3 4 5
Output
6
Input
5 12 13
Output
30
Input
45 28 53
Output
630
输入格式
见题面
输出格式
见题面
样例输入
3 4 5
样例输出
6