def yes_or_no():
    """The user must answer with Y or N. Returns a bool"""
    answer = None
    while answer is None:
        answer = input().upper()
        if answer == 'Y':
            answer = True
        elif answer == 'N':
            answer = False
        else:
            answer = None
            print('Invalid input. Your answer must be Y or N')
    return answer
