Normal sudoku rules apply. Cells marked with a circle show the number of digits with the same parity as that circled digit in that cell's 3x3 neighbourhood (including the digit in the cell itself). (Eg if r1c1 is a 1, all 3 cells surrounding it must be even.) Digits cannot appear in the same relative position within different 3x3 box regions. (Eg r1c1 and r4c4 cannot contain the same digit as they both represent the top left cell of their 3x3 boxes.)