#include <bits/stdc++.h>
using namespace std;
int main(void) {
  int a, l, x;
  double h, s;
  while (cin >> a >> l >> x) {
    s = sqrt(a * a * l * l / 4.0 - a * a * a * a / 16.0);
    h = sqrt(pow((l + x) / 2.0, 2) - l * l / 4.0);
    s += h * l;
    printf("%.10f\n", s);
  }
}
