The ornamental design for a scarf, as shown and described.
FIG. 1 is a right-side view of the scarf . The left side is a mirror image.
FIG. 2 is a front view of the scarf shown in FIG. 1.
FIG. 3 is a rear view of the scarf shown in FIGS. 1 and 2.
FIG. 4 is a rear view of the scarf before it is tied around a wearer's head; and,
FIG. 5 is a side view of the scarf as shown in FIG. 4.
The single broken line shown at the base of the claimed design in FIG. 4 is stitching. The broken lines showing a person's head and neck in the drawings are meant to depict the design while in a condition of use, and form no part of the claim.