#include <iostream>
using namespace std;
int main(){
	long long A, B;
	cin >> A >> B;
	if (A != B){
		cout << (A * B + 1) / 2 - 1 << endl;
	} else if (A % 2 == 0){
		cout << (A / 2) * (A / 2) - 1 << endl;
	} else {
		cout << (A / 2) * (A / 2 + 1) << endl;
	}
}