The ornamental design for a desk, as shown and described.
FIG. 1 is a perspective view showing the rear, left side, and top of a desk showing my new design;
FIG. 2 is a left side elevational view thereof; and,
FIG. 3 is a front elevational view thereof.
The right side elevational view is a mirror image of the left side. The bottom view is not shown because it forms no part of the claimed design. The casters are shown in broken lines for illustrative purposes only and form no part of the claimed design. The writing surface is formed of transparent material.