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 shaded areas represent boundaries of the claimed design while all other broken lines are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.