Wow! Really cool and well thought through game concept! I think the UI needs a bit more clarity to get in at the beginning, e.g. replace the "0/12$" with "12$", and write "remaining" under it, which makes it more intuitive. Also the "E" under erase looks like a "C", which confused me. If you now add some SFX and cool 8-bit music this would be top-class! I enjoyed it! :)