Normal sudoku rules apply. Outside clues are the absolute difference between the sum of digits between 7 and 8, and the sum of digits between 8 and 9, in their corresponding row or column. '?' represents a digit (0~9) to be determined by the solver. All sandwiches have at least one filling, i.e. there are no runs of three adjacent cells in any row or column containing only the digits 7, 8, and 9.