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

using ll = long long;

int main(void) {
	ll N, M;
	cin >> N >> M;
	
	if(N == M) cout << N * N / 4 + (N % 2) - 1 << "\n";
	else cout << (N * M + 1) / 2 - 1 << "\n";
	
	return 0;
}