This app is a Japanese abacus (soroban/そろばん). In the free version, you can use the abacus as much as you want. However, to challenge yourself, you can use the abacus in the challenge mode.

In the challenge mode, you have four operations: addition, subtraction, one- and two-digit multiplication, and division. You also have three difficulty levels: easy, medium, and hard. You must compete against the clock to get the highest score.

This app content tutorials basic concepts and operations. It is simple and fun to use. It is completely free, so you can enjoy it without spending any amount of money.