#include<iostream>
#include<cstdio>
#include<complex>

using namespace std;

double area(double a, double b) {
  return sqrt((a / 2 + b) * (-a / 2 + b) * a * a / 4);
}

int main() {
  double a, l, x;
  while (cin >> a >> l >> x) {
    printf("%.12f\n", area(a, l) + area(l, (l + x) / 2) * 2);
  }
  return 0;
}