The goal is to order consecutive numbers, starting with 0,
from left to right and from top to bottom, with a low score.
When a number tile is ordered, mark the tile (tap or click).
You may demark a marked tile, but the score increases.
With each move (swipe or arrow key), a new number tile
appears in a random cell and the score increases.
When a tile bumps into its neighbor, the tile disappears,
while the neighbor's number increases (UP & RIGHT)
or decreases (DOWN & LEFT).
Mark the last cell to see your score, the lower the better.