I claim the ornamental design for a desk, as shown and described.
FIG. 1 is a front elevational view of a desk embodying my new design;
FIG. 2 is right side elevational view thereof, the left side elevational view being a mirror image thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a bottom plan view thereof, the broken lines shown in this figure are for environmental purposes only and form no part of the claimed design.