13+14. Matt in einem Zug
White's captures are g6xNf7 and h6xBg7. Black captures are c5xd4,
b5xc4 and a7xb6, in this order. So the white a-pawn promoted and is
still on the board. The only white piece that can go to a8 and unpromote
is the knight on g8.
Suppose black has the move. Then the retroplay is:
-1. Ba7-b8 Bb8-c7 -2. Rc7-c6 Rc6-d6 -3. f3-f4 Rd6-e6 -4. Re6-e7
Ne4-g5 -5. Re7-e6 Ng3-f4 -6. Re6-e7 Nf5-g3 -7. Re7-e6 h4-h3 -8.
Re6-e7 Ne7-f5 -9. Nh6-g8 Ng8-e7 -10. Re7-e6 Re6-d6 -11. Nf5-h6 Rd6-c6
-12. Rc6-c7 Bc7-b8 -13. Ne3-f5 Bb8-c7 -14. Nc2-e3 Bc7-b8 -15. Nb4-c2
Bb8-c7 -16. Na6-b4 Bc7-b8 -17. f2-f3 Bb8-c7 -18. Nc7-a6 h5-h4 -19. Na8-c7
Bc7-b8 -20. Bb8-a7 h6-h5 -21. a7-a8=N and black is retro-pat.
With white to play, the retro-play is almost similar, except that
black doesn't have to lose a valuable pawn tempo early in the retro-play,
because, due to the changed tempo, Ng8-e7 can get retracted without
the pawn retraction earlier.
So white has the move: #1 with 1. Bc7#
Solution by mr.mip