The Magician and his Assistant show trick. The Viewer writes on the board the sequence of N digits. Then the Assistant covers some pair of adjacent digits so that they become invisible. Finally, the Magician enters the show, looks at the board and guesses the covered digits and their order. Find the minimal N such that the Magician and his Assistant can agree in advance so that the Magician always guesses right combinatoricsnumber theorygamegame strategy