No.660(Answers)

Black 1 is crucial.
If White 2, Black 3 and 5.
White has to create a Ko with 6.(one-move approach Ko for White)
If White 2 at 3, Black 2 kills White.

If White 2, Black 3.
White has to create a one-move-approach Ko with 4.

Black 1 is crucial.
If White 2, Black gets a Ko with 3.