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