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 elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a right side elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines immediately adjacent to the shaded areas depict the bounds of the claimed design, while all other broken lines are directed to portions of the article that form no part of the claimed design.
The wire frame lines shown throughout the views are intended to indicate surface contour.