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: Wrong Answer)

#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define f(i, x, n) for(int i = x; i < (int)(n); ++i)
#define speed ios::sync_with_stdio(0); cin.tie(0);
int const N = 1e5 + 2;
struct P
{ 
	int x, y;
	void sc(){cin >> x >> y;}
}p[N];
bool op(P p1, P p2){return p1.x >= p2.x && p1.y > p2.y;}
int main(){
	speed
	int n, k;
	cin >> n >> k;
	f(i, 0, n)p[i].sc();
	sort(p, p + n, op);
	ll sum = 0, c = 1;
	f(i, 0, k)sum +=p[i].y;
	f(i, 1, k)if (p[i].x != p[i - 1].x)c++;
	cout << sum + (c * c) << endl;
}