Normal sudoku rules apply. The grid contains nine "three-sum" cells, one in each row, column, and 3x3 box. Each digit from 1-9 is a "three-sum" exactly once. “Three-sum” digits count as their nominal value increased by 3. For example, a “three-sum” 7 would have a value of 10. The sum of the value of the digits on a line within a box (the line's 'region sum') must be the same for each box that line passes through.