Normal sudoku rules apply. A clue outside the grid shows the sum of the indicated diagonal, which may include repeats. Each digit (1-9) has a "buddy" that appears in all of the cells where the row and column indices are swapped. For example, if 1's are buddies with 4's and there is a 1 in r5c2, then there must be a 4 in r2c5. A number may be its own buddy. Figuring out the buddies is part of the puzzle.