The ornamental design for a cup, as shown and described.
FIG. 1 is a perspective view of a cup showing our new design;
FIG. 2 is a front view of the cup of FIG. 1;
FIG. 3 is a rear view of the cup of FIG. 1;
FIG. 4 is a side view of the cup of FIG. 1; and,
FIG. 5 is a top view of the cup of FIG. 1.