Problem Statement

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.

Input

Input is given from Standard Input in the following format:
|AB| |BC| |CA|

Output

Print the area of the triangle ABC.

Constraints

• $1 \leq |AB|,|BC|,|CA| \leq 100$
• All values in input are integers.
• The area of the triangle ABC is an integer.

Sample Input 1

3 4 5

Sample Output 1

6

This triangle has an area of 6.

Sample Input 2

5 12 13

Sample Output 2

30

This triangle has an area of 30.

Sample Input 3

45 28 53

Sample Output 3

630

This triangle has an area of 630.

见题面 Input 部分

见题面 Output 部分