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 $ABC$.

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 $630$.