This is an archived problem from a previous year. Submissions are closed, but you can view the problem and its solution.
After solving the checkless helpmate on the ancient chessboard, you climb back to the surface, your mind still pondering the possible connection between the mysterious letters, the torn Santa mask, and Maskenissen's disappearance. As you climb out the door integrated into the f6 square on the giant chessboard from day 7, you run into Peter Swindler, the Elf known around the North Pole for dealing used cars.
You tell him about everything you've seen, about the letters, the torn Santa mask. Swindler listens, then says with a sly smile, "Well well well, quite the adventure you're on. I happen to know who's behind those letters. Solve the following problem, and I'll tell you."
He opens the trunk of his car, and after shuffling aside a box labeled 'GENUINE NORTH POLE SNOW, TOTALLY NOT STYROFOAM,' he brings out a chessboard.
"Starting from the initial position, construct a position where you mate with the rook that starts on a1 on the 5th move," he says.
"Easy," you think, reaching to play 1. a4 to quickly bring the rook into action. Swindler stops you and pushes the pawn back to a3. "Not so fast! You must begin with 1. a3. Other than that, all standard rules apply."
In short: Construct a game where White starts with 1.a3 and mate Black on their 5th move with their queen side rook. Regular chess rules apply: White and Black alternates moves. Check out Offerspill's Discord server if you're confused.