50A Domino piling

#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <string>
#include <algorithm>
using namespace std;

int main() {

    int m, n;
    cin >> m >> n;
    cout << m*n/2 << endl;

    return 0;
}