1...f5
Diagram ...f7-f5 must have been the last move! The white pawns have made ten captures, and ten Black pieces (and pawns are off the board. Hence, all of Black's pieces were captured by the white pawns... So we know that the Black Bc8 moved out earlier to be captured by a white pawn (The Black Bc8 was NOT captured on c8). Because we know that Black moved the d-pawn at an earlier time, Black's last move must have been f7-f5.
2.gxf6
And mates with f6-f7 on the next move *