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>
#define INF 1e18
#define MOD 1000000007
#define PI M_PI
#define ll long long
#define vi vector<int>
#define all(v) v.begin(), v.end()
#define FOR(i, a, b) for (int i=a; i<b; i++)
#define REP(i, n) for (int i=0; i<n; i++)
#define REPD(i, n) for (int i=n-1; i>=0; i--)
#define print(x) cout << x << endl;
#define debug(x) cout << #x << " = " << endl;

using namespace std;

void solve()
{
    int a, b, c;
    cin >> a >> b >> c;

    print(a * b / 2);
}

int main()
{
    solve();

    return 0;
}