#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
#include <string>
using namespace std;



int main() {
	long long a, b,ans;
	cin >> a >> b;
	if (a == b) {
		if (a % 2 == 0) ans = a*b / 4;
		else ans = a*b / 4 + 1;
	}
	else {
		if (a % 2 == 1 && b % 2 == 1) ans = a*b / 2 + 1;
		else ans = a*b / 2;
	
	}
	cout << ans - 1 << endl;
	return 0;
}