This is a simple but challenging puzzle. To solve it you have to put the numbers in the tiles in order, from 1 to 9.
To change the number in the tile just tap it to highlight it and the use the "+" and "-" buttons to increase or decrease the number. The numbers will not decrease lower than 1 or increase higher than 9.
The catch: When you change a number in a tile, the tiles to the left, right, top and bottom of it will also change.
Try to solve it in as few moves as possible.