Normal sudoku rules apply. Digits in a cage must sum to the given total. Octopus rules apply. I.e. draw 4 lines that must each: i) not cross or overlap anywhere except r5c5, where they all intersect; ii) have 180-degree rotational symmetry around r5c5; iii) visit exactly 2 cells on the edge of the grid, which are the beginning and end of the line; and iv) all be the same length (in number of cells passed through). The 4 lines are "Same Difference" lines. I.e pairs of adjacent digits on a line must all have the same difference, though this difference may be different for different lines.