The user does not enter the numbers with the smartphone keyboard, but with the buttons below the Sudoku grid.

For the number keys, a short (normal) keystroke writes the number to the selected field. A long keystroke writes the number into the field as a small hint for the user. In this case several numbers can be written into one field. With the delete key, a normal keystroke will delete all digits in the selected field, a long keystroke will delete only the digit to the left of the cursor.

The goal is to write the numbers from 1 to 9 once in each row, each column, and each 3x3 subgrid of the 9x9 Sudoku grid. If the user presses the buttons "New Game" (a new Sudoku grid is loaded) or "Solution" (the solution of the current Sudoku is displayed), the current game is considered lost.