The most difficult sudoku in the world

Sudoku is the most easily recognized and played puzzle game in the world. In 2012 mathematician Arto Inkala said he created the most difficult to solve Sudoku. You can try it yourself and see if this is actually true.

 

What is the figure in place of red question? Register on your website and write the answer as a comment below the post.

For those of you who do not know how to solve a Sudoku, the goal is to fill all the squares with numbers from 1 to 9 and there are 3 simple rules:

1. In each 3×3 square must have the numbers 1 to 9 without repeating any.
2. In any line of the large square may have the numbers 1 to 9 without repeating.
3. In each column of the large square can may have the numbers 1 to 9 without repeating.
   

Pleasant conundrum :).