The ornamental design for a pen, as shown and described.
FIG. 1 is a front perspective view of a pen showing my new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a front elevational thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a rear elevational view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view.
The broken lines in FIGS. 1 to 8 are for environmental purposes only and form no part of the claimed design. The stippling shown on the pen represents shading and is not part of the claimed design.