#include <bits/stdc++.h>
using namespace std;
long double f(int x, int y) { return x * sqrtl(y * y - x * x / 4) / 2; }
int a, l, x;
int main() {
  while (cin >> a >> l >> x) {
    cout << fixed << setprecision(20) << f(a, l) + f(l, (l + x) / 2) * 2
         << endl;
  }
}
