The answer is 226, which is the record for a straight mate-in-something problem with no alternative moves for White at any stage of the solution. (It's up to 257 if you don't mind the occasional second solution).
The solution has the elegance of a knitting pattern: 1.Bh2+ Kf3 (best) 2.Qe3+ Kg2 3.Qg1+ Kf3 4.Qf1+ Kg4 (Ke4 allows Qd3 mate) 5.Qe2+ Rf3 6.Qe6+ Rf5 7.Nd7! Any Black knight move invites a quick mate with Ne5 or Nf6+, so Black plays 7...Kf3!
White must not now be tempted by 8.Qxf5+? Kg2, when Black is doing well, but runs through the sequence again starting 8.Qe3+ Kg2 9.Qg1+ and ending 12.Qe6+ Rf5. After 13.Kb2! Kf3, 19.Kc1 Kf3 and 25.Kd1 (the missing moves are the usual checking sequence) Black cannot play Kf3 because of Qe2 mate. So he pushes 25...a3 and White must return with 26.Kc1, 32.Kb1, 38.Ka2 and 44.Kxa3.
White plods back for 62.Kd1 a4! 63.Kc1 Kf3, more checks, then 69.Kd1 again. Black temporises with a5 and a6, then must play a3. After losing all his a-pawns, Black forces the final tour with c4, and the solution ends when he runs out of pawn moves and must meet 224.Kd1 with 224...Nhf8 225.Nf6+ Kf3 226.Qe2 mate!Reuse content