In 2002-2003, Russian mathematician Grigory Perelman published on the Internet a proof of the Poincaré conjecture - the “millennium problem”, for the solution of which the American Clay Institute awarded a million dollar prize. It took time for the mathematical community to recognize Perelman's achievement. However, the scientist was never able to present the award: he refused it, just as he had previously refused the prestigious Fields Medal. Several years ago, Perelman announced that he was no longer studying mathematics and had minimized contacts with the outside world. Famous journalist and writer, deputy editor-in-chief of the Snob project Masha Gessen explores the Perelman phenomenon, relying on numerous interviews with his teachers, classmates and colleagues . A special place in the book is given to the history of Russian math schools, which have educated more than one generation of remarkable scientists and simply thinking people. "Perfect Severity" is Gessen's first book published in Russian.