Meisterwerke

White's only hope for win is 1.b5xc6 ep. This will need a proof that black's last move was c7-c5. The proof goes as follows:

White captures have been c2xb3, e2xd3xc4xb5 and f2xe3xd4xc5xb6. These 8 captures account for all missing black pieces, including the two promoted at f1 from black g- and h-pawns.

Black captures include axbxa to explain the a2-pawn and gxf, hxgxf by pawns on their way to promotion. So all black and white captures were by pawns.

A further consideration in the diagram position is providing a return path for the black king. For example white can retract c2xb3 or d3xc4 only after black king has managed to escape to the fourth row. The other problem is that white has to be careful not to block the f-file (by retracting f2xe3) before black pawns are unpromoted at f1 and returned beyond f2-square.

Because of the above considerations black's objective in the retroplay is either to quickly return the king to the fourth row or provide white with more moving men by an uncapture. It proves out that the former task is impossible without blocking the f-file, while the second succeeds with following retractions :

1. .. c7-c5 2. c5xRb6 Rf6-b6 3. d4xc5 Rf1-f6 4. e3xd4 f2-f1=R 5. c4xb5 g3xQf2 6. Qf6 etc, etc. Now that white has an extra piece, there is all the time in the world to unpromote the second pawn and wriggle out with the king via d4. If black starts retractions with 1 .. c6-c5 (or any other move) instead, it takes too long to reach f1 for the uncaptured piece.

So 1. .. c7-c5 is the only possible last move for black and starting the solution with en passant is justified.

Solution by mr.mip