Normal sudoku rules apply. Cells are initially covered in fog, and placing correct digits will clear the fog around those digits. Mark nine cells such that there is one marked cell in each row, column and 3x3 box. The digit in the first cell of a row, column or 3x3 box gives the position of the marked cell in that row, column or 3x3 box. Each digit appears in exactly one marked cell. Adjacent digits on a green line differ by at least 5.\n\nEg if r5c3 was a marked cell then r5c1 would be a 3 (marked cell is 3rd position in its row); r1c3 would be a 5 (marked cell is 5th position in its column); and r4c1 would be a 6 (marked cell is the 6th position in normal reading order of its 3x3 box).