There is a group of n people, for every person in this group there is a list of people he/she likes. One of the people has a book that everybody wants to read. Every person can lend the book to other person, but only if he/she likes this person. You need to find out, is it possible to organize transfer of the book between people in such a way, that everybody will have it exactly ones and in the end it will return to its owner? Represent this problem as an algorithmic problem. What is this algorithmic problem? Is it possible to solve it in a polynomial time