It's actually very simple. If you don't know how the code works, try to follow it through a few examples, then everything should be clear. There are a few tricks used here.

I just wanted to say that building applications that are resistant to unexpected user use requires writing more code.

How many fewer lines of code would we have if users typed what we wanted? 😉

Yes, you're right, although I thought I was just giving an idea.

Mind the input validation.

The more you know, the less you will do 🙃

Thanks 😉

You're right, I understood it over time.

This may not be the most optimal solution, but it is consistent with the content of the task:
Deem yourself brilliant if you can come up with a version that does not use any function whatsoever.
The author probably wanted you to think a little deeper about the solution.

Mathematics in programming is very helpful.

According to the description, where is my money? 😉