Could someone please explain me what does phrase "Rocks of pairwise different colors" mean? I assume that in second example the only possible set of Rocks is "ababa", but I can see there 6 pairs of Rocks with different colors. The only possible match to the samples I can see is if you could pick a set of Rocks with at most k different colors, but I don't think it is the right meaning of the statement.

Update: Yep, my last guess was correct after all.

Edited by author 25.12.2014 00:30

Yep, I don't understand the phrase either. Please fix the problem statement. :)