Thank you, SoloLearn!
It is 6 months after I have first opened this amazing app. It gave me a powerful impact in learning how to code. I still remember passing tests on Java. As a result, I have published my first app in Play Store — https://play.google.com/store/apps/details?id=com.niksatyr.mathtrainerpro One more time, thank you a lot for such a wonderful app!