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<vector>
#include<algorithm>
#include<cmath>
#include<utility>
#include<stack>
#include<queue>
#include<map>
#include<set>

using namespace std;

#define FOR(i, a, n) for(int i = (int)(a); i < (int)(n); ++i)
#define REP(i, n) FOR(i, 0, n)

#define PRINT(str) printf(#str "\n")
#define yOUT PRINT(Yes)
#define nOUT PRINT(No)
#define YOUT PRINT(YES)
#define NOUT PRINT(NO)
 
#define pb(a) push_back(a)
#define all(x) (x).begin(),(x).end()

int main(){
	int a, b, c;
	cin >> a >> b >> c;

	cout << a*b/2 << endl;
}