The ornamental design for a desk, as shown and described.
FIG. 1 is a front perspective view of a desk showing my new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a right side elevational view thereof, a left side elevational view not being shown as it is a mirror image of the right side elevational view; and,
FIG. 5 is a top plan view thereof.
The broken lines are for environmental purposes only and form no part of the claimed design.