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

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	long long r, c;
	cin >> r >> c;
	long long ans = 0;
	if(r == c) {
		if(r % 2 && c % 2) {
			ans = (r * c - 1) / 4 + 1;
		} else {
			ans = (r * c) / 4;
		}
	} else {
		if(r % 2 && c % 2) {
			ans = (r * c - 1) / 2 + 1;
		} else {
			ans = (r * c) / 2;
		}
	}
	cout << ans - 1 << endl;
	return 0;	
}