#include <iostream>
#include <cstdio>

using namespace std;

int r, c;

int main() {
    // freopen("abacus.in", "r", stdin);
    // freopen("abacus.out", "w", stdout);

    scanf("%d%d", &r, &c);

    printf("%lld\n", (1LL * r * c + 1) / 2 - 1);

    return 0;
}