Help with challenge for Overriding Methods in Object-Oriented Swift 2.0

I'm not sure what I'm meant to be doing in this challenge and was wondering if anyone might be able to help. The challenge is as follows and my code is attached.

I've provided a base class Person in the editor below. Once an instance of Person is created, you can call getFullName() and get a person's full name.

Your job is to create a class named Doctor that overrides the getFullName() method. Once you have a class definition, create an instance and assign it to a constant named someDoctor.

For example, given the first name "Sam", and last name "Smith", calling getFullName() on an instance of Person would return "Sam Smith", but calling the same method on an instance of Doctor would return "Dr. Smith".

I know I have a lot to learn in this area to be able to understand what I'm doing but if anyone can help on this challenge so I can learn a little more I'd be very grateful.

Thanks,

Nikki

class Person {
    let firstName: String
    let lastName: String

    init(firstName: String, lastName: String) {
        self.firstName = firstName
        self.lastName = lastName
    }

    func getFullName() -> String {
        return "\(firstName) \(lastName)"
    }
}

// Enter your code below

class Doctor: Person {

    override func getFullName() -> String {
    return "\("Dr.") \(lastName)"
}}


let someDoctor = Doctor(firstName: "Sam", lastName: "Smith")