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

double heron(double a, double b, double c) {
  double s=(a+b+c)/2;
  return sqrt(s*(s-a)*(s-b)*(s-c));
}

int main() {
  double a,l,x;

  while(cin>>a>>l>>x)
    printf("%.6f\n", heron(a,l,l)+2*heron(l,(l+x)/2,(l+x)/2));
  
  return 0;
}