The ornamental design for a brush, as shown and described.
FIG. 1 is a perspective view of a brush from a lower right side perspective showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken line showing of the brush head is included for the purpose of illustrating a portion of the article that forms no part of the claimed design.