#include <bits/stdc++.h>
using namespace std;
int main() {
int a, b, pro;
cin >> a >> b;
pro = (a*b);
cout << (pro/2);
return 0;
}
I2luY2x1ZGUgPGJpdHMvc3RkYysrLmg+CnVzaW5nIG5hbWVzcGFjZSBzdGQ7CgppbnQgbWFpbigpIHsKCWludCBhLCBiLCBwcm87CgljaW4gPj4gYSA+PiBiOwoJcHJvID0gKGEqYik7Cgljb3V0IDw8IChwcm8vMik7CglyZXR1cm4gMDsKfQ==