The ornamental design for a desk, as shown and described.
FIG. 1 is a perspective view of a design for a desk showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front elevational view thereof,
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof; and,
FIG. 6 is a rear elevational view thereof.
The dashed lines indicate the cross members that can be used to provide structural support for the desk of the present invention; namely of those portions of the desk which do not form a part of the design.