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<iostream>
#include<set>
using namespace std;

int NextNum( int num )
{
	if ( num % 2 == 0 ) {
		return num / 2;
	}
	else {
		return 3 * num + 1;
	}
}

int main() {
	int s;
	cin >> s;

	int next = s;
	set<int> nums;
	nums.insert( next );

	int idx = 1;
	while ( true ) {
		next = NextNum( next );
		pair<set<int>::iterator, bool> res =
			nums.insert( next );
		if ( res.second ) {
			cout << idx + 1 << endl;
			return 0;
		}
		++idx;
	}

	return 0;
}