Mathematics with Boris Trushin. Combinatorics: from scratch to Olympiads

From the author: «Hello everyone! My name is Boris Trushin, and I am a mathematics teacher. I have been teaching mathematics for 24 years and for more than six years I have been running a fairly popular YouTube channel «Boris Trushin» on school-related mathematics... In this book, I have gathered my many years of experience in teaching combinatorics to students of different ages. I have tried to show a path that anyone can follow if they want to understand the basics of this science. If you are just starting to get interested in this topic, read the book from the very beginning, stopping and trying to solve all the problems presented here. Don't be upset if not everything works out, put the problem aside for a day or two and think about it more. At the end of the book, there are detailed solutions to each problem. You will learn to understand combinatorics at a fairly high level. Enjoy your reading!»