Introduction: the most widespread variants of sudoku can be broadly grouped into two categories:
1) The variants that add a new constraint, like the diagonals where you should also include numbers 1 to 9 in the main diagonals or the nonconcecutive where two consecutive numbers can not be adjacent.
2) The variants that add or completely replace the initial information (in the classic sudoku is just the number already entered in the cell), as the killer sudoku and greather than sudoku .
You can also think of other types of variants, for example, those who do not use all 9 numbers but only a subset of the numbers themselves.

Origin: Adolfo Zanellati
Rules - Regole:
you must put in each row, column, block only the numbers 1, 2, 3 and 4 repeated 2 times and 1 cell for each row, column, block should be left empty (-).
dovete inserire in ogni riga, colonna, settore i numeri 1, 2, 3 e 4 ripetuti 2 volte ; 1 cella per ogni riga, colonna, settore deve essere lasciata vuota (-).