#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(n);++i)
#define all(a) (a).begin(),(a).end()
#define dunk(a) cout << (a) << endl
using namespace std;
typedef long long ll;

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	int r, c;
	cin >> r >> c;

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

	return 0;
}