Game Description: Sum Sudoku 9x9 Hard. Each puzzle consists of a 9x9 Sudoku grid containing areas surrounded by gray or dotted lines. The object is to fill all empty squares so that the numbers 1 to 9 appear exactly once in each row, column and 3x3 box, and the sum of the numbers in each area is equal to the clue in the area’s top-left corner. In addition, no number may be used in the same area more than once.