#include <bits/stdc++.h>
using namespace std;
#define GET_MACRO(a, b, c, NAME, ...) NAME
#define rep(...) GET_MACRO(__VA_ARGS__, rep3, rep2)(__VA_ARGS__)
#define rep2(i, a) rep3 (i, 0, a)
#define rep3(i, a, b) for (int i = (a); i < (b); i++)
#define repr(...) GET_MACRO(__VA_ARGS__, repr3, repr2)(__VA_ARGS__)
#define repr2(i, a) repr3 (i, 0, a)
#define repr3(i, a, b) for (int i = (b) - 1; i >= (a); i--)
template<class T1, class T2> inline bool chmin(T1 &a, T2 b) { return b < a && (a = b, true); }
template<class T1, class T2> inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }
using ll = long long;

int main() {
	ll r, c;
	cin >> r >> c;

	ll ans;

	if (r == c) {
		// ##...
		// ##...
		// ###..
		// .....
		// .....
		if (r % 2 == 0) {
			ans = (r / 2) * (c / 2);
		} else {
			ans = (r / 2) * (c / 2) + (c + 1) / 2;
		}
	} else {
		// #####
		// ###..
		// .....

		// #####
		// #####
		// .....
		// .....
		if (r % 2 == 0) {
			ans = (r / 2) * c;
		} else {
			ans = (r / 2) * c + (c + 1) / 2;
		}
	}

	cout << ans - 1 << endl;

	return 0;
}