The ornamental design for a brush, as shown and described.
FIG. 1 is a front perspective view of the brush, which embodies my design;
FIG. 2 is a front side view of the brush;
FIG. 3 is a rear view of the brush;
FIG. 4 is a left side view of the brush;
FIG. 5 is a right side view of the brush;
FIG. 6 is a top view of the brush; and,
FIG. 7 is a bottom view of the brush.
The dash-dot-dash broken lines shown in FIGS. 1, 6, and 7 represent the bounds of the claim and form no part thereof. The dashed broken lines along with the dash-dot-dash broken lines illustrate portions of the brush that from no part of the claimed design.