This is a really rough one. I've been playing Cytus since around 2017. They have somewhat recently decided to add ads in between playing songs. This is an incredible disappointment to me as Cytus stood out from other mobile apps not only because of the quality but also because of the fact there was no monetization. If you do enjoy this game enough as I did, there is a Nintendo Switch version of the game out there for 60 USD. All chapters included, including ones you must pay on the app to play.