Incredibly smooth game - the music and UI animations were just lovely. I really liked the design and the encounters that force you to rely solely on the minus operator.
One small hiccup early on: I initially thought I had to place the number on the operator space itself (though that might just be on me xD)
While playing, I had an idea - maybe on certain levels, players should have to subtract exactly the given number, with a penalty for going below. Right now, it sometimes feels a bit too easy to win.