#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  while (getline(cin, s)) {
    stringstream ss;
    ss << s;
    int a, l, x;
    ss >> a >> l >> x;
    auto r = l / 2.0 * sqrt(x * x + 2.0 * l * x) +
             a / 4.0 * sqrt(4.0 * l * l - a * a);
    cout << setprecision(11) << r << endl;
  }
}
