A knight stands on the top left square of a 2n×2n chessboard. In one move the knight goes 1 square in one direction and 2 squares in the perpendicular direction, and it may not leave the board. It may step on a square it has already visited.
Count the ways to move the knight at least 0 and at most k times so that it ends on one of the four corner squares of the board. Two ways are different if the number of moves differs, or if the sequence of visited squares differs anywhere. The starting square is already a corner, so making no move at all counts as one way.