The ornamental design for a stamp, as shown and described.
FIG. 1 is a top perspective view of a stamp showing our new design;
FIG. 2 is a front elevation thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a side elevation thereof, the side opposite being a mirror image;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a perspective view of a modified embodiment of the design shown in FIGS. 1 through 5;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a side elevation thereof, the side opposite being a mirror image.
The front elevation, top plan, side elevation and bottom plan aspects of the modified embodiment shown in FIGS. 6 through 8 are identical to what is shown in FIGS. 2, 3, 4 and 5. The broken lines shown in the drawings represent environment and form no part of the claimed design.