Hello Codeforces,

On behalf of the organizers of the Polish Olympiad in Informatics (OI), I am happy to invite you to participate in an online mirror contest based on the problems from the final round of this year's edition of the Olympiad.

The mirror contest will take place on March 18–19, 2026, each day from 16:00 to 21:00 (CET).

Participants will solve the original tasks from the OI finals (with statements in English) in a contest environment. The allowed programming languages are C++ and Python, with model solutions written in C++.

Before the contest, practice problems will be available from March 17 at 18:00 until March 18 at 09:00 (CET) so that participants can familiarize themselves with the system.

The contest will be hosted on the SIO2 platform: https://sio2.mimuw.edu.pl/c/oi33-3-mirror

Everyone interested in practicing algorithmic problem-solving is warmly welcome to participate!

The mirror contest is sponsored by Hectonet — many thanks for their support.

Hello codeforces!

In one of the recent contests there appeared a problem which involved Euler's phi function, and I figured I could write a bit about this function here.

First, I will briefly introduce Euler's phi function, talk about two of its properties, and give you proofs of both of them. Ultimately, it will lead us to a way to compute the value of this function for some given integer $$$n$$$ with reasonable time complexity.