#include <iostream>

using lint = long long;

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

    if (n == m) {
        std::cout << std::max(0LL, (n * m + 1) / 4 - 1) << std::endl;
    } else {
        std::cout << (n * m + 1) / 2 - 1 << std::endl;
    }
}

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

    solve();

    return 0;
}