The ornamental design for a pen, as shown and described.
FIG. 1 is a perspective view of a pen in accordance with the invention.
FIG. 2 is a front plan view thereof.
FIG. 3 is a rear plan view thereof.
FIG. 4 is a left plan view thereof.
FIG. 5 is a right plan view thereof.
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The features in broken lines on the top and bottom in FIGS. 6 and 7 are included for the purpose of showing portions of the pen that form no part of the claimed design.