I didn't know how to solve it during the round, and solving it requires coming up with a non-trivial hypothesis (that the only operation we need to do with regard to the canals is to take some prefix, level it, then join with our region) and either proving or trusting it. This hypothesis did not even occur to me, so I was quite far from solving it. I think this problem is conceptually much harder than the fourth one.

My view is that the last two problems have similar difficulty levels, but he had to make a choice of which problem to think about (it doesn't do you any good attacking multiple problems at once), so he didn't think a lot about the last one. It's not something you see instantly, but after doing some cases by hand you eventually come to the prefix conclusion. (I'm operating under the assumption that if I could see it, then Petr would see it too).

Well, initially I was quite certain it would pass, but the solution turned out a bit more complicated and thus slower. Also, I'm coding much much slower in C++ and thus might not have finished the problem in time. And finally, I'm not certain that C++ would help here — the slow operation is long division (needed to multiply two longs with "cap"), and that's probably implemented simiarly in C++ or Java. Maybe using a 64-bit compiler would help more actually :)