Boris Trushin has been teaching mathematics to schoolchildren and students for almost 25 years, is a co-author of school textbooks on algebra and has been running the YouTube channel of the same name on near-school mathematics for 7 years. The author's second book smoothly immerses the reader in number theory and allows you to master the basics of this interesting section of mathematics without any prior knowledge. Number theory problems are often found at mathematical olympiads and the Unified State Exam. You will go through an exciting path from the very basics, you will understand where the multiplication properties came from and why the column division algorithm works, and end with Euler's theorem.