kosic.50:
What about response T2 after S1: S1, T2; R1, R2; N1, P2; Q1 seki

sitruun4:
There is another solution for the last move. 1. S1, Q2 2. R1, T2 3. N1, O1 4. R2! and white can't approach from either side and is captured. There is only one ko threat left in the position either way, so there is really no meaningful difference in the two solutions ( the other being 4.Q1)

JN7975:
C18 as White's last move works too