#include<bits/stdc++.h>
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 a1=heron(a,l,l);
    double a2=heron(l,(l+x)/2,(l+x)/2);
    printf("%.10f\n",a1+a2*2);
  }
}