#include <iostream>
#include <cstdio>
#include <cmath>
using namespace std;

double area(double a, double b, double c)
{
  double s = (a+b+c)/2.0;
  return sqrt(s * (s-a) * (s-b) * (s-c));
}

int main()
{
  int a, l, x;
  double s0, area0;
  double sa, area_slack;

  while (cin >> a >> l >> x)
  {
    area0 = area(a, l, l);
    area_slack = area(l, (l+x)/2.0, (l+x)/2.0);
    printf("%.10f\n", area0 + 2 * area_slack);
  }
  return 0;
}