#include <bits/stdc++.h>
using namespace std;

int main() {
    long long R, C;
    cin >> R >> C;

    if (R == C) {
        cout << R * C / 4 - !(R % 2 && C % 2) << endl;
    } else {
        cout << R * C / 2 - !(R % 2 && C % 2) << endl;
    }
}