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 <math.h>
#include <stdlib.h>
#include <string.h>
#include <algorithm>
#include <array>
#include <iomanip>
#include <iostream>
#include <limits>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>

using namespace std;

#define rep(i, n) for (int i = 0; i < (n); i++)
#define loop(i, a, n) for (int i = a; i < (n); i++)
#define all(in) in.begin(), in.end()
const int INF = 1e9;
const int MOD = 1e9 + 7;
int main() {
    int a, b, c;
    cin >> a >> b >> c;
    cout << a * b / 2 << endl;
    return 0;
}