Ensure that your program handles invalid input, such as negative age or an invalid ticket type.

Sure, I can help with that. Here's a step-by-step guide on how to handle invalid input in a program:

1. Input Validation: The first step is to validate the input. This means checking if the input meets certain conditions. For example, if you're expecting an age, you should check if the input is a positive number. If you're expecting a ticket type, you should check if the input matches one of the valid ticket types.

2. Error Messages: If the input is invalid, you should inform the user with an appropriate error message. This could be something like "Invalid age. Please enter a positive number." or "Invalid ticket type. Please enter one of the following: ...".

3. Re-prompting: After displaying the error message, you should prompt the user to enter the input again. This could be done in a loop that continues until the user enters a valid input.

4. Handling Exceptions: In some cases, the input might cause an exception to be thrown. For example, if you try to convert a string to an integer and the string does not represent a valid integer, an exception will be thrown. You should catch these exceptions and handle them appropriately, usually by displaying an error message and re-prompting for input.

Here's a simple example in Python:

while True:
    try:
        age = int(input("Please enter your age: "))
        if age < 0:
            raise ValueError("Age cannot be negative.")
        break
    except ValueError as e:
        print(e)

while True:
    ticket_type = input("Please enter your ticket type: ")
    if ticket_type not in ["adult", "child", "senior"]:
        print("Invalid ticket type. Please enter one of the following: adult, child, senior.")
    else:
        break

In this example, the program will keep asking for the age and the ticket type until the user enters valid inputs.