#include <bits/stdc++.h>
using namespace std;
double heron(double a, double b, double c) {
  double s = (a + b + c) / 2.0;
  return sqrt(s * (s - a) * (s - b) * (s - c));
}
int main() {
  double a, l, x;
  cout << setprecision(10);
  while (cin >> a >> l >> x) {
    cout << heron(l, l, a) + 2 * heron(l, (l + x) / 2.0, (l + x) / 2.0) << endl;
  }
  return 0;
}
