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 left side elevational view of FIG. 1.
FIG. 4 is a right side elevational view of FIG. 1.
FIG. 5 is a rear elevational view of FIG. 1.
FIG. 6 is an enlarged top plan view of FIG. 1; and,
FIG. 7 is an enlarged bottom plan view of FIG. 1.
The broken lines showing features in the top view of FIG. 6 are included for the purpose of showing portions of the pen that form no part of the claimed design.