Simple, A little math used in a unique way, nice.

The first level is blank, there is only a number and the menu, but nothing else.

I disliked the very beggining, because there was no interaction other then watching how you are supposed to do things... to me a good tutorial lets the player interact with the game (albeit in a controlled space) to know the gimmick. And the first 5 levels serve as a better tutorial than those cutscenes at the beggining.
Other than that I really liked the rest. The idea of the game was interesting, the execution was good and you always added some new aspect to it in a good pace. And the levels were clever, but because I had no ideai how many there were I just stopped at 26.
Overall it's a good game. Good job!

this is a very clever game

I like it. The music doesn’t do it for me, but the concept does. It gradually introduces each mechanic (though the early tutorial screens could do with some more flare so they aren’t mistaken for the next puzzle) and the difficulty curve slopes up nicely. It has some of those puzzle design moments where you feel really cleaver when you find the solution, but sometimes you happen upon the solution by accident. No way to fix that though.

Also, if you want it to be truly relaxing, tell people how many levels there are.