It might have been my bad for trying to engage with the game quite late in the day, but it might have been a bit too much reading for me and I didn't get the game. After reading through some of the comments here however I get what the game is about a bit more. It's an interesting idea. I think given that blue is subtraction actually putting -X rather than just X value would have helped communicate that a bit clearer. At the start I was confused on what the target score was so it might be an idea to break up the text tutorial with some visuals, even screenshots might do you some good.