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