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 C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
typedef long long LL;
using namespace std;
#define INF (1LL << 60)
#define ALL(x) (x).begin(),(x).end()
#define FOR(i,k,n) for(int i=(k);(i)<(n);(i)++)
#define REP(i,n) for(int i = 0;(i)<(n);(i)++)
#define REPR(i,n) for(inti = n; i >= 0; i--)

int main() {
	int a, b, c;
	cin >> a >> b >> c;
  	cout << a * b / 2;
}