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At first sight, it seems that it can simply be solved using disjoint set ds. But the problem is more complicated because we handle two kinds of sets at once. Can anyone give a hint for this problem? (PS : I've already try to write and proof my solution for 3 days but still get WA. I suspect I didn't cover all the possible cases in the problem. And sorry if I'm not explaining my solution. It's too messed up I don't want to use it anymore.)