Score : $100$ points

Problem Statement

There is a right triangle $ABC$ with $\angle ABC=90^\circ$.

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 $ABC$.

3 4 5

6

This triangle has an area of $6$.

5 12 13

30

This triangle has an area of $30$.

45 28 53

630

This triangle has an area of $630$.