Addition simulator

App for all ages, to train columnar addition. Having two numbers, this app checks every single step of their addition, highlights the current column, and immediately signals if a calculation error occurs.
Currently there are two modes:
- normal, where you input the addends manually;
- random, where the addends are generated automatically.
For your convenience, you can choose a background of the app.
In future versions oа the app, operational modes will be extended, as well as the overall app functionality.