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 view thereof;
FIG. 5 is a right view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof;
FIG. 8 is another perspective view thereof; and,
FIG. 9 is still another perspective view thereof.
The broken lines shown in the views are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.
The wire frame lines shown throughout the views are intended to indicate surface contour and are not intended to be surface ornamentation.