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.

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 $ABC$.