Abdel Irada: ∞
<<+> Say! Whose file *is* this, anyway? <+>>
What a pickle our knight is in. He's attacked by a pawn, and moreover, if the pawn takes him, it will do so with dangerous threats involving ...Qxh2+, because as anyone can see, Black controls the open h-file, so often decisive in kingside attacks.
So, we should obviously retreat the knight, right?
Of course not. This is a puzzle; whatever the solution is, the key move is *not* going to be 23. Nf3.
So, how about saccing him? We could mess up Black's kingside with 23. Ne6+, or even 23. Nf5+, and sometimes breaking up the pawn structure can lead to a break*through*.
But there's better: Ignore the threat and toss a rook at f7. After all, it's a puzzle, yes?
Well, okay. The real reason is that removing that pawn entices the king, brings the queen into the game with gain of tempo, and (perhaps most importantly) destroys the protection of the important square e6 from that knight still patiently standing on d4.
<<+> 23. Rxf7+! ... >
We will begin with the acceptance of the sacrifice as usual:
1) <<+> 23. ...Kxf7 >
This is the most natural response. Any other simply loses time and material for no compensation, although analysis will show whether it might prolong the game.
<<+> 24. Qd5+ ...>
Here Black is best advised to avoid moves that let White bring the rook to f3 with check, so
1.1) <<+> 24. ...Kg7>
is essentially forced.
(Not so good are
1.2) <24. ...Ke7?/...Ke8? 25. Qe6+, Kf8 26. Rf3+, Kg7 27. Qe7+>, with mate to follow, or
1.3) <24. ...Kf8? 25. Rf3+, Kg7> (25. ...Ke7/e8? 26. Qe6#) <26. Qe7+>, mating as above.)
<<+> 25. Ne6+ ...>
And I hope you enjoy that femur in your royal trachea!
Now the king must move. He has five options.
1.1.1) <25. ...Kh6?? 26. Rh3#>
1.1.2) <25. ...Kh7 26. Rh3+, Kg8 27. Ng5+, Kg7/f8 28. Qf7#>
1.1.3) <25. ...Kg8 26. Ng5+, Kg7 27. Qf7+, Kh6 28. Rh3+!, Kxg5 29. Qe7+>, with mate in two.
1.1.4) <25. ...Kf7? 26. Ng5+> wins by transposition to one of the preceding lines. (Also, 26. Nxd8++ is not bad.)
1.1.5) <<+> 25. ...Kf6!>
This is a clever attempt to turn the femur into a stumbling block, because the knight occupies the square White's queen craves.
<<+> 26. Rf3+, Ke7
27. Ng5 ...>
What else? We can't afford to take the rook at the cost of ending our attack while still behind on material, so we create threats on f7 and e6. These can be parried with only one move:
<<+> 27. ...Rdf8
28. Rf7+, Rxf7
29. Qe6+!, Kd8 >
Not 29. ...Kf8?? 30. Qxf7#.
<<+> 30. Nxf7+, Kc7 >
Worse is 30. ...Kc8 31. Nxh8 : winning advantage to White, with an extra pawn, safer king, and more active pieces.
<<+> 31. Qd6+, Kc8
32. Qxb8+, Kxb8
33. Nxh8, Nf8
34. Nf7, Nd7
35. Kf2 >
White is a solid pawn up with a better pawn structure, while Black will find it hard to hold his e- and g-pawns for long. He can pick up White's b-pawn in exchange for the e-pawn, but that leaves his knight far away on b2 when White gets his kingside pawns rolling with h4 (followed perhaps by attacking the now-immobilized g-pawn). This leads to a race which White wins handily.
Now let's look at declining the rook. There are two ways to do this.
2) <23. ...Kh6?
24. Rh3+, Kg5
25. Ne6+, Kg4
3) <23. ...Kg8
24. Rxd7 >
White is a piece ahead. And Black can't get it back with 24. ...Rxd7 25. Qxd7, exd4? because of 26. Re8+.
It looks as though Black can escape (in variation 1.1.5) into a probably lost ending. Or he can try a different variation and lose faster.
Unless I'm going completely chess-blind and missed some hidden depths, this isn't really an "insane"-level puzzle.