I'm glad to invite everybody to take part in the current round.
Today the problems' author is me. I with Artem Rakhov, Maria Belova and Dmitry Matov have tried to do everything possible to make problems more reliable.
I hope that the problems will be interesing for you, the contest will be dinamical, and stronger competitors will cope with the problems quickly to help newcomers to catch all bugs by their hacks :)
P.S. The phrase "stronger competitors will cope with the problems quickly" in no way means that all the problems will be easy. Don't forget to read statements and to test your solutions!
UPD. The contest is over. Congratulations to the winner -
UPD2. Tutorial is added. In spite of the record number of participants the system has been working stable during the round. Thanks to Codeforces team! Also thanks to everyone who took part in the round.