thanks

What's not forbidden, can be used. Not a kata issue, closing.

I don't see why is that an issue. Do you see anywhere in the description that it shouldn't be used?

OP solved it, closing.

The easiest way to see the difference is to do both solutions for some smaller inputs by hand.
If you're confused about the function you've used, try to write it out as a loop and compare the differences.

nobody will know what you think is missing unless you tell them

Try some examples on paper.

have you TRIED IT?

Not a kata issue. See https://docs.codewars.com/training/troubleshooting#post-discourse.

I must be too stupid to understand the problem.

but why

I see two things that can be changed:

To make it easier... as you do the mental calculations in the water you either freeze when you realise you are dead, or swim when you realise you can make it!

that's redundant, and, halve is misspelled.

I'm guessing you mean something completely different, but how would anyone know what you're thinking when you don't say?