class Vehicle {
    let make: String
    var speed = 0.0

    init(make: String) {
        self.make = make
    }

    func describe() -> String {
        "\(make) traveling at \(speed) km/h"
    }
}

class Car: Vehicle {
    var numberOfDoors: Int

    init(make: String, numberOfDoors: Int) {
        self.numberOfDoors = numberOfDoors
        super.init(make: make)
    }

    override func describe() -> String {
        super.describe() + " with \(numberOfDoors) doors"
    }
}

let car = Car(make: "Toyota", numberOfDoors: 4)
car.speed = 80
print(car.describe())   // Toyota traveling at 80.0 km/h with 4 doors