The ornamental design for a pen, as shown and described.
FIG. 1 is a perspective view of a pen showing my new design.
FIG. 2 is a front elevational view of FIG. 1.
FIG. 3 is a right side elevational view of FIG. 1, the left side elevational view being a mirror image thereof.
FIG. 4 is a rear elevational view of FIG. 1.
FIG. 5 is a top plan view of FIG. 1.
FIG. 6 is a bottom plan view of FIG. 1; and,
FIG. 7 is an exploded perspective view of FIG. 1.
The broken lines in FIGS. 1-7 showing the cap, writing tip housing, and end opposite from the writing tip are included for the purpose of illustrating portions that form no part of the claimed design.