Interesting idea and I had a lot of fun once I got the hang of it. I started doing a lot better when I realized my goal was basically to get as many unique numbers on the board as possible. My best score was 32. I'd probably pay a dollar or two for an upgraded version of this on my phone.
My main nitpick is one of my biggest pet peeves, the arial font. I think basically any other font looks better. I guess one other thing is music might of been nice but I didn't really mind the lack of music that much, there's solid sound feedback for all the main elements of the game which is the main thing that matters. Also I'd maybe consider doing absolute value for subtraction, getting a negative number is basically a dead tile in your board most of the time.