The ornamental design for a stamp, as shown and described.
FIG. 1 is a perspective view of a stamp, showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The circular broken lines immediately adjacent to the claim depict the bounds of the claimed design, while all other broken lines are directed to environment. In addition, the unshaded surface that directly adjoins the claimed solid line edges forms no part of the claimed design.