#include <bits/stdc++.h>
using namespace std;
double calc(double a, double b, double c) {
  double s = (a + b + c) / 2;
  return sqrt(s * (s - a) * (s - b) * (s - c));
}
void func(double a, double l, double x) {
  double tmp = (l + x) / 2;
  printf("%.10lf\n", calc(a, l, l) + calc(tmp, tmp, l) * 2);
}
int main() {
  double a, l, x;
  while (scanf("%lf %lf %lf", a, l, x) != EOF) {
    func(a, l, x);
  }
}
