#include <bits/stdc++.h>
using namespace std;
int main() {
double a, t;
cin >> a >> t;
double luas;
luas = 0.5 * a * t ;
cout << fixed << setprecision(2);
cout << luas << endl;
}
I2luY2x1ZGUgPGJpdHMvc3RkYysrLmg+Cgp1c2luZyBuYW1lc3BhY2Ugc3RkOwoKaW50IG1haW4oKSB7CiAgICBkb3VibGUgYSwgdDsKICAgIGNpbiA+PiBhID4+IHQ7CiAgICAKICAgZG91YmxlIGx1YXM7CiAgIAogICBsdWFzID0gMC41ICogYSAqIHQgOwogICAKICAgY291dCA8PCBmaXhlZCA8PCBzZXRwcmVjaXNpb24oMik7CiAgIGNvdXQgPDwgbHVhcyA8PCBlbmRsOwp9CiAgICA=