I have been trying to write this code and I need a way of stopping the code if a certain input is incorrect, here is what I have so far:

```
first_name = input('PLEASE ENTER YOUR FIRST NAME: ')
last_name = input('PLEASE ENTER YOUR SURNAME: ')
print(f'Hello {first_name} {last_name}, before you enter.')

def age_det():
    age = input(f'How old are you?')
    converted_age = [int(age)]

    for num in converted_age: 
      while num>=18:
        print(f'Awesome, {first_name}, welcome to Blablabla')
        num += 100
      while num <= 18:
        break
      print(f'Sorry, {first_name}, but we require you to be at least 18 to enter Blablabla.')
      num += 100

age_det()
#I want the code to stop here if the age entered is under 18    

username = input('Before we start, please pick a username: ')

print(f'Woah! {username}, good choice!')

```