The book contains a variety of information about the number of PIs - the famous mathematical constant that appears in a wide variety of places. The main part of the book has an informative and entertaining character. It sets out information available to a wide range of mathematics lovers. The additional part of the book provides solutions and answers to the problems formed in the main part, as well as reference data and comments that partially go beyond the school course, but do not go beyond the standard course of higher mathematics at the university.