#include <iostream>
#include <cmath>
using namespace std;

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

	if (r == c){
		cout << ((long long)r * c - 1) / 4 << endl;
	}
	else{
		cout << ((long long)r * c - 1) / 2 << endl;
	}

	return 0;
}