The Game of 15 is a popular puzzle game invented in 1878 by Noah Chapman. It is a set of identical square knuckles with numbers applied, enclosed in a square box. The length of the side of the box is four times the length of the side of the knuckles for a set of 15 elements, respectively, one square field remains empty in the box. The goal of the game is to move the knuckles around the box, to arrange them by numbers, preferably by making as few movements as possible.

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