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This is an archived problem from a previous year. Submissions are closed, but you can view the problem and its solution.

23

RetractorRetrogradeHelpmate

As you free Wesley in the chair from the ties and removes the cloth from his mouth, he shouts, "GRINCHUK!", pointing to the other Wesley.

David Snowell and Magnus Carolsen grab the Wesley who's been accompanying the group and pull off his mask, revealing the smirking face of Grinchuk. Grinchuk looks around the room for any possible escape, but he's surrounded.

"Ah, what a show we've had!" Grinchuk exclaims. "After my defeat 100 years ago I've been lurking in the shadows, planning a comeback tour. I knew to strike at the heart of Christmas, I needed to take down Maskenissen. No Maskenissen, No Christmas! But you," he points at you, "turned out to be a problem I didn't expect."

Grinchuk explains how he laid out chess puzzles across the North Pole. He even replaced some of Santa's reindeer with his own, making them lure you into his cave, away from the elves' protection. "But those elves and their bell changed your course, led you to that church and away from my trap," he sneers.

He admits to watching you play against Wesley Snow at the pub and, in a moment of opportunity, knocking out the real Wesley and assuming his identity. "I had to be on the inside to make sure you failed. Yet, puzzle by puzzle, you kept solving them, foiling my plans. I tried to guide you astray, but here we are," Grinchuk sighs, a twisted sense of admiration in his tone. "But you're still too late to save Christmas," he grins. "Maskenissen is hidden in an undergroung cave."

The group forces Grinchuk to lead them to the underground cave. It's the same underground cave where you faced some of the initial challenges.

Finally, you arrive at a barred-off section of the cave. Behind the bars, you see Maskenissen, his once-jolly eyes now filled with despair.

Grinchuk says, "You want to save him? Then you must solve one final problem. Solve it, and the key to Maskenissen's cell is yours."

In the current position, White has just completed a move. First, retract the last move made by White, followed by retracting the last move made by Black. Then, resume the game by making a legal move for Black, and finally, a move for White. White's last move should deliver checkmate.

You do not need to provide the two retracted half-moves in the solution, only the two moves being played forward after the retraction. First a Black move, then a White move, delivering checkmate.

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First 10 to solve #23