Rules: Fill the grid with digits 1-4 so that each digit appears exactly once in every row, column and every 3×3 spiral. Digits should be placed orderly in the spirals, from the entrance to the center. There cannot be any digits in the cells marked with “-“.
Background: This type is one of my inventions. As I remember correctly, I had an intend to make some new Sudokus for 4th WSC in Zilina. Probably I found the idea at that time. Anyway I used this puzzle first time in OAPC 2, then TLSCBWSC, appeared in Team Quarter Final of 18th WPC. Also used by Zoltan Horvath in 20th WPC.