#include <iostream>

using namespace std;

int main() {
	long long r, c, ans;

	cin >> r >> c;
	if (r != c) {
		ans = (r * c + 1) / 2 - 1;
	} else {
		if (r % 2 == 0) {
			ans = r * c / 4 - 1;
		} else {
			ans = ((r + 1) / 2) * (c / 2) + 1 - 1;
		}
	}

	cout << ans << endl;
	return 0;
}