The ornamental design for a pen, as shown and described.
FIG. 1 is a front perspective view of a pen according to a third embodiment of my design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The evenly spaced broken lines are included to show parts of the pen that form no part of the claimed design. The long and short broken lines in FIGS. 1-5 that define the bounds of the claimed design form no part thereof.