Problem:
Score : 100 points

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.

Solution in PyPy3 (2.4.0) (judge verdict: Accepted)

# -*- coding: utf-8 -*-
"""
A - Right Triangle
https://atcoder.jp/contests/abc116/tasks/abc116_a

"""
import sys


def solve(a, b, c):
    s = (a + b + c) / 2
    return int((s*(s-a)*(s-b)*(s-c))**0.5)

def main(args):
    a, b, c = map(int, input().split())
    ans = solve(a, b, c)
    print(ans)


if __name__ == '__main__':
    main(sys.argv[1:])