The ornamental design for a desk, as shown and described.
FIG. 1 is a front elevational view of a desk showing my design;
FIG. 2 is a rear elevational view of the desk;
FIG. 3 is a right side elevational view of the desk;
FIG. 4 is a left side elevational view of the desk;
FIG. 5 is a top plan view of the desk;
FIG. 6 is a bottom plan view of the desk; and,
FIG. 7 is an enlarged perspective view of an upper portion of the desk.
Throughout the Figures, portions represented by broken lines are not parts of the claimed design; the dot-and-dash lines merely show the boundaries between the claimed parts to be protected and the other parts; and the parallel thin lines on surfaces represent surface shading and do not illustrate ornamentation or decoration.