#include <iostream>
#include <cstdio>
#include <cmath>
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){
    double t = (l+x)/2;

    printf("%.10f\n", heron(a,l,l)+heron(t,t,l)*2);
  }

  return 0;
}