Logic Masters Deutschland e.V.

A while back I discovered a series of cool puzzles by Richard with a ruleset I loved. Here are links to Richard's amazing City Construction Puzzles:

• City Construction #1
• City Construction #2
• City Construction #3
• City Construction #4
• City Construction #5 Labyrinth

Earlier I made a puzzle that adapted the spiraling rule to digits from 1-9: Stepping Stones Now here's a puzzle that adds my favorite genre of sandwich. Hope you enjoy!

The Rules

1. Normal sudoku rules apply.
2. [Stepping Stones] Spiraling in clockwise starting from the top corner, circle/shade the first 1 you see, then the first 2 you see after that, and then the first 3 you see after that, etc. After you've reached 9, continue again by circling the next 1 you see, and the next 2... etc. In the final solution, there must be exactly 2 circled digits in every row, column, and box.
   • ex: if R1C3 is 1 and R1C4 is 2, they should be circled. If R1C8 is then 3, the puzzle is broken because there would be three circled cells in Row 1.
3. [Sandwich Variant] Numbers on the outside of the grid represent the sum of the digits sandwiched between the two circled digits in every row, column.

The Puzzle: Try it on CTC's Sudokupad (with answer check enabled.)