[ikarous]

The problem's description is deceptively simple. It poses at the end a combinatorics problem that is non-trivial. I submitted a correct solution after a bit of research; my first approach was the naive solution.

[tootlol]

How did you submit the solution with a library? Doesn't upload mechanism assume only 1 file?

[helpmehn]

If, by "non-trivial," you mean NP-hard, I agree. :-)

[ikarous]

I was trying to be vague so as not to give hints to those interested in trying the challenge. Evidently, I am utterly guileless.

[helpmehn]

Given the number of combinatorial optimization problems that are NP-hard, I don't think that gives anything away to anyone who doesn't already know what you're talking about.