Where is the loophole in my code

[micseydel]

Anyway, maybe I should give up on this question and save time for moving on new challenges and readings

That's very possible. Efficiency is an advanced topic, and has enough math that it might be worth developing more programming experience before trying to tackle it. It's definitely worth learning at some point, but I was programming for ~5 years without any issues before learning it. It can be very fun once you do learn it though, and the basics of efficiency only require arithmetic. Advanced efficiency topics, such as analyzing recursive functions, can require inductive proofs which are a more advanced kind of math.

In any case, whichever way you choose will probably be fine :)