The goal of the game is to solve a 4-digit puzzle generated by the game app. The puzzle consists of four Arabic digits from 0 to 9, with no repetition of the digits.

The feature of this app allows you not to be constrained by the left-to-right order when thinking about the answer. The 4 gray squares in the middle of the screen represent 4 digits of the guess. Touch any square to make a "question mark (?)" appear and then touch the blue number key below. The tapped number will appear in the "?"-marked gray square.

The gray squares that have already been filled in can also re-enter new digits. When all 4 digits are confirmed, press the "ENTER" key, and the app will tell you the result of this guess.