def add(x, y):
    return x + y

def subtract(x, y):
    return x - y

def multiply(x, y):
    return x * y

def divide(x, y):
    if y == 0:
        return "Error! Division by zero."
    else:
        return x / y

def calculator():
    while True:
        print("Options:")
        print("Enter 'add' for addition")
        print("Enter 'subtract' for subtraction")
        print("Enter 'multiply' for multiplication")
        print("Enter 'divide' for division")
        print("Enter 'exit' to end the program")
        user_input = input(": ")

        if user_input == "exit":
            print("Exiting the program.")
            break

        if user_input not in ["add", "subtract", "multiply", "divide"]:
            print("Invalid input. Please try again.")
            continue

        num1 = float(input("Enter first number: "))
        num2 = float(input("Enter second number: "))

        if user_input == "add":
            print("Result:", add(num1, num2))
        elif user_input == "subtract":
            print("Result:", subtract(num1, num2))
        elif user_input == "multiply":
            print("Result:", multiply(num1, num2))
        elif user_input == "divide":
            print("Result:", divide(num1, num2))

calculator()