The 15-puzzle (also called Gem Puzzle, Boss Puzzle, Game of Fifteen, Mystic Square and many others) is a sliding puzzle that consists of a frame of numbered square tiles in random order with one tile missing.
The puzzle also exists in other sizes, particularly the smaller 8-puzzle.

The object of the puzzle is to place the tiles in order from first to last tile by moving the tile next to the empty space into it.

puzzle can be:
3x3 (8-puzzle or 9-puzzle),
4x4 (15-puzzle or 16-puzzle),
5x5 (24-puzzle or 25-puzzle) or
6x6 (35-puzzle or 36-puzzle)

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