#include <iostream>

using lint = long long;

void solve() {
    lint n, m;
    std::cin >> n >> m;

    int sim = (n == m ? 4 : 2);
    std::cout << (n * m + sim - 1) / sim - 1 << std::endl;
}

int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    solve();

    return 0;
}