#include <bits/stdc++.h>
using namespace std;
int main() {
  double a, l, x;
  while (cin >> a >> l >> x) {
    double dM = l * sqrt(x * x + 2 * l * x) * 0.5;
    double A = a * sqrt(l * l - a * a * 0.25) * 0.5;
    cout << dM + A << endl;
  }
  return 0;
}
