#include <iostream>

using namespace std;

int main() {
	long long r, c;
	cin >> r >> c;
	long long count;
	if (r == c) {
		if (r % 2 != 0) {
			count = (r * r - 1) / 4 + 1;
		} else {
			count = r * r / 4;
		}
	} else {
		if (r % 2 != 0 && c % 2 != 0) {
			count = (r * c - 1) / 2 + 1;
		} else {
			count = r * c / 2;
		}
	}
	cout << (count - 1) << endl;
	return 0;
}