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 <iostream>
#include <math.h>
#include <numeric>
#include <vector>
#include <map>
#include <algorithm>
#include <queue>
#include <tuple>
#include <functional>

#define PI 3.14159265359
#define INF 100100100100100100
#define rep(i,n) for(int i = 0; i < n; i++)
#define rep2(i,a,n) for(int i = a; i < n; i++)
#define in(a) cin >> a
#define out(a) cout << a << endl

using namespace std;

typedef long long ll;
typedef pair<ll, ll> PLL;

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

    return 0;
}