The ornamental design for a stamp, as shown and described.
FIG. 1 is an exploded perspective view of a stamp showing my new design in an open position;
FIG. 2 is an exploded top plan view of the stamp in an open position;
FIG. 3 is a top plan view of the stamp in a closed position;
FIG. 4 is an exploded front elevation view of the stamp in an open position.
FIG. 5 is an exploded rear elevation view of the stamp in an open position;
FIG. 6 is an exploded left side elevation view of the stamp in an open position; the right side elevation view of the stamp is a mirror image of the left side elevation view; and,
FIG. 7 is a bottom plan view of the stamp in a closed position.
The outermost broken lines immediately adjacent to the claimed edge represent the bounds of the claimed design while all other broken lines are directed to environment; the broken lines form no part of the claimed design.