#include <bits/stdc++.h>
#define p(s) cout<<(s)<<endl
typedef long long ll;
using namespace std;

int main() {
	ll R,C;
	cin>>R>>C;
	ll ans;
	if (R == C)	ans = (R * C + 3) / 4;
	else ans = (R * C + 1) / 2;
	p(ans-1);
	return 0;
}