Normal sudoku rules apply. The clues in the grid correspond to sandwich clues of the same letter on the outside of the grid. Each clue in the grid is either a 2 or 1 digit number, read left to right or up down and must be determined by the solver. Clues outside the grid ("sandwich clues") show the sum of the digits "sandwiched" between the 1 and the 9 in that row/column.