#include <bits/stdc++.h>
using namespace std;
int main() {
  long double a, l, x;
  cout << std::setprecision(13);
  while (!cin.eof()) {
    cin >> a >> l >> x;
    long double h_1 = l * l - a * a / 4.0;
    long double h_2 = (x + l) * (x + l) / 4.0 - l * l / 4.0;
    cout << a * sqrtl(h_1) / 2 + sqrtl(h_2) * l << endl;
  }
}
