Normal sudoku rules apply. There are 18 doubler cells in the grid - 2 in each row, column and 3x3 box, and they must contain exactly 2 sets of digits 1-9. Those cells are considered to have double their value for the purposes of the arrow clues, in which values along an arrow sum to the value in the attached circle. A clue outside the grid gives the sum of the digits between the two doublers in that row or column.