#include <bits/stdc++.h>
using namespace std;

int64_t divceil(int64_t a, int64_t b) {
	return a / b + (a % b ? 1 : 0);
}

int main() {
	int64_t R, C;
	cin >> R >> C;
	cout << (divceil(R * C, 2) - 1) << endl;
	return 0;
}