My idea was not original
I made a sad discovery that what I thought was my original idea in the "20 Hops" game has been done before. In fact, as far as 7 years ago (or 11 years ago?), TANSTAAFL developed "Kight's Challenge" and put it on Game Development Showcase. His game comes with nice graphics, animation, sound effects, and a bigger board. So, moving forward, I need to put more effort on product differentiation, such as trying to come up with a friendly hint system.
I implemented undo. Download Version 1.02.