Beginner: Dear Farkas.
I just got the answer.
the last step doesn't have to be at that point.
the black can take any other move. I ask you to consider about it.
Because in this problem settings, if the black doesn't go to the very exact last step, it says 'incorrect.'
Farkas: I couldn't follow your comment. Consider what? There are 20 accepted last moves.
:)