Bad kata. We have none of instructions in description. Like already said in comments, if you are good searcher on web you will find what you need to do..

I totally agree. Same here. Got stuck 2 hours of "finding error", I've got so pissed, so decided to write all combinations on paper, and check it. Should be no mistake. Someone needs to check Kotlin random tests.

I wrote a solution which works on IDE, but somehow on random test it have mistake, and when i try that exact problem on IDE, it works perfectly.