Fifteen puzzle – popular puzzle game invented in 1878 by Noah Chapman. Represents a set of identical square dominoes are wearing numbers, enclosed in a square box.
The length of the box four times greater than the length of the dominoes to a set of 15 elements, respectively, in the box remains blank one square box. Goal of the game – moving the knuckles on the box to organize them by number, preferably by making the least possible displacement and in less time.
Exactly half of all possible initial positions can not result in assembled form. Therefore, the game offers only solvable positions.

Moving the knuckles on the box to organize them by number.