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 thereof.
FIG. 3 is a right side elevational view thereof; the left side elevational view being a mirror image thereof.
FIG. 4 is a rear elevational view thereof.
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof.
The short broken lines are included for the purpose of illustrating portions of the pen that form no part of the claimed design. The long and short broken lines define the bounds of the claimed design and form no part thereof.