The goal is to fill all the field. You have to pass exactly n times on
each case, where n is the printed number.

When you go on a case, the number is decremented. When it's 0, it
becomes a wall.

Keys:
  . s, d, e, f or arrow keys: move left/down/up/right. Press enter to
    confirm.
  . h: print this help file
  . q: exit the game
  . v: instant victory