• Normal Sudoku rules apply: enter the digits 1–9 in each row, column and 3×3 box.

• Each row has a skyscraper sum, which is the sum of the digits ‘visible’ from the left — digits represent the height of skyscrapers and taller skyscrapers obstruct the view of smaller ones behind them (to the right). For example, the row 1 6 3 5 7 9 2 8 4 would have the skyscraper sum of 1+6+7+9 = 23.

• Each column has an X-sum, which is the sum of the first X digits, where X is the first digit (X is itself included in that sum). For example, the column 5 7 2 9 4 8 1 3 6 would have the X-sum 5+7+2+9+4 = 27.

• The skyscraper sums (of the rows) and the X-sums (of the columns) must progress in strictly ascending or descending order. Furthermore, neighboring values cannot be consecutive (e.g., a 12 next to a 13 is not allowed).

• Where a “forbidden sandwich” clue is provided, the sandwich sum in that row/column MUST NOT equal that clue. The sandwich sum is the sum of the digits sandwiched between the 1 and the 9 (not inclusive). For example, the row 6 3 1 2 7 5 9 8 4 would have the sandwich sum of 2+7+5 = 14.