public override double cena()
 {
     if (car is Mercedes)
     {
         return(car.cena() + 60000);
     }
     else if (car is Ford)
     {
         return(car.cena() + 20000);
     }
     return(0);
 }
 public override double cena()
 {
     return(car.cena() + 8000);
 }