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

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

int main(){
  double a,l,x;
  while(cin >> a >> l >> x) printf("%.10f\n",heron(a,l,l) + heron(l,(l+x)/2.0,(l+x)/2.0)*2.0);
}