Fifteen - a puzzle, which is a 15 square tiles with printed numbers from 1 to 15. All the knuckles are enclosed in a square box of 4x4 size (the side of the square box is four times longer than the knuckles). Thus, when placing the knuckles in the box, there is one empty space the size of one knuckle, which can be used to move the knuckles inside the box. The goal of the game is to arrange the placement of numbers in the box, placing them in ascending order from left to right and from top to bottom, starting with the knuckle number 1 in the upper left corner and ending with an empty space in the lower right corner of the box.