In the life of every young person, the day inevitably comes when he has to take a math exam. Is it possible to help a youth survive this day, and not just survive, but more or less successfully move on to the next? That is, pass the exam. Is there a recipe for success? In authoritative opinion, the exam is nothing more than a combination lock, and to open it, you need to know the cipher. But the peculiarity of this castle is that there are many ciphers: not the only sequence of solved problems leads to the goal, and many, many and many. But... All such sequences have something in common.