Command.swift

class Calculator {
    private (set) var total = 0
    
    func add(amount: Int) {
        total += amount
    }
    
    func subtract(amount: Int) {
        total -= amount
    }
    
    func multiply(amount: Int) {
        total = total * amount
    }
    
    func divide(amount: Int) {
        total = total / amount
    }
}

let calc = Calculator()
calc.add(10)
calc.multiply(4)
calc.subtract(2)
print("Total: \(calc.total)")