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 <iomanip>
#include <random>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <cmath>
#include <numeric>
#include <cstdlib>
#include <cstring>
#include <deque>
#include <sstream>
#include <bitset>
#include <cassert>
#include <fstream>
#include <queue>

#define len(X) ((int)(X).size())

#ifdef __LOCAL
	#define DBG(X) cout << #X << "=" << (X) << '\n';
#else
	#define DBG(X)
#endif

using std::cin;
using std::cout;
using std::vector;
using std::string;
using std::pair;
using std::swap;
using std::sort;
using std::lower_bound;
using std::unique;

using int64 = long long int;
using pii   = pair<int, int>;
using pii64 = pair<int64, int64>;

const int NIL = -1;
const int INF = (int)2e9;
const int64 INF64 = (int64)2e18;

void init_IO() {
	std::ios_base::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);
}

void solve() {
	int64 ab, bc, ca;
	cin >> ab >> bc >> ca;

	cout << (ab * bc) / 2 << '\n';
}

int main() {
	init_IO();
	solve();

	return 0;
}