#include <bits/stdc++.h>
using namespace std;
int main() {
  double a, l, x;
  double s;
  while (cin >> a >> l >> x) {
    x += l;
    s = sqrt(l * l - a * a / 4.0) * a / 2.0;
    s += sqrt(x * x / 4.0 - l * l / 4.0) * l;
    cout << s << endl;
  }
}
