//59
#include<iostream>
#include<cmath>

using namespace std;

int main(){
  for(double a,l,x;cin>>a>>l>>x;){
    double s=l+x/2;
    cout<<fixed<<sqrt((l+a/2)*a/2*a/2*(l-a/2))+2*sqrt(s*(s-l/2-x/2)*(s-l/2-x/2)*x/2)<<endl;
  }
  return 0;
}