#include <bits/stdc++.h>
using namespace std;

int a, b, c;

long double solve();

int main() {
  cin >> a >> b >> c;
  cout << fixed << setprecision(10);
  cout << solve() << endl;
  return 0;
}

long double solve() {
  long double s = (a + b + c) / 2.0L,
              area = sqrtl(s * (s - a) * (s - b) * (s - c));
  return area / 4;
}