What is the hardest problem ever used in the contest of Codeforces?

Revision en1, by phuthuynhochienthan, 2015-10-14 12:02:00

I wonder which problem in the contest of Codeforces is the hardest one.

When I sort the problem list of Codeforces base on number of Accepted submission of user, I find out some problems as follow:

111E - Petya and Rectangle 68E - Contact

What is your opinion? :D

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en1 English phuthuynhochienthan 2015-10-14 12:02:00 351 Initial revision (published)