The ornamental design for a brush, as shown and described.
FIG. 1 is a perspective view of a brush showing my new design;
FIG. 2 is a top view thereof;
FIG. 3 is a left view thereof front view thereof, with the right side being a mirror image;
FIG. 4 is a front view thereof;
FIG. 5 is a rear view thereof; and,
FIG. 6 is a bottom view thereof.
The broken lines immediately adjacent the surface shading on the handle represent the bounds of the claim while the broken lines showing bristles and parts of the handle illustrate environmental structure none of which forms part of the claimed design.