#include <bits/stdc++.h>
using namespace std;
int main() {
  while (!cin.eof()) {
    double a, l, x;
    cin >> a >> l >> x;
    double h_1 = sqrt(l * l - (a / 2) * (a / 2));
    double m = a * h_1 / 2;
    double h_2 = sqrt(((x + l) / 2) * ((x + l) / 2) - (l / 2) * (l / 2));
    double s = l * h_2 / 2;
    printf("%.6f\n", m + s * 2);
  }
  return 0;
}
