#include <iostream>
using namespace std;

int main() {
    long long R, C, K;

    cin >> R >> C;

    if (R != C) {
        K = R * C / 2 + (R * C) % 2 - 1;
    }
    else {
        K = R * C / 4 + R % 2 - 1;
    }

    cout << K << endl;

    return 0;
}