#include <bits/stdc++.h>
using namespace std;
int main() {
  double a, l, x;
  while (scanf("%lf%lf%lf", &a, &l, &x), a) {
    double s1 = (a + l + l) / 2, s2 = (l + l + x) / 2;
    double d = (l + x) / 2;
    printf("%.10lf\n", sqrt(s1 * (s1 - a) * (s1 - l) * (s1 - l)) +
                           sqrt(s2 * (s2 - d) * (s2 - d) * (s2 - l)) * 2);
  }
}
