The rules of the sudoku game are:

The starting grid is a 9x9 matrix that has some numbers that can`t be changed by the user. This numbers are placed in such a manner so that the puzzle`s solution is unique.
Example:

puzzle

On each line there should be only one appearance of a digit(1-9)

Example: On the following row you already have the numbers 2,3,4 and 7 so you can`t add one of this digits in the blank boxes.
first row

On each column there should be only one appearance of a digit(1-9)

Example:On the following column you already have the numbers 3,5,8,6,9 and 7 so you can`t add one of this digits in the blank boxes.

second column

In each highlighted 3x3 grid there should be only one appearance of a digit(1-9)

Example:In the following 3x3 box you have the numbers 4,5,9 and 7 so you can`t add this digits in the blank boxes.

: fifth box

The starting digits that are present on the grid are not changeable and the user should add their numbers around this digits

The game ends when all the fields are completed respecting all the previous rules

Usually completing a puzzle takes anywhere from 10 to 30 minutes,but may take longer when trying a hard puzzle

This is how a completed puzzle looks like:

solution