The ornamental design for a pen, as shown and described.
FIG. 1 is a front side perspective view of my new design with the phantom lines depicting environmental structure and forming no part of the claimed design;
FIG. 2 is a front elevational view thereof with the phantom lines depicting environmental structure and forming no part of the claimed design;
FIG. 3 is a right side elevational view thereof which is a mirror image of the left side thereof with the phantom lines depicting environmental structure and forming no part of the claimed design;
FIG. 4 is a back elevational view thereof with the phantom lines depicting environmental structure and forming no part of the claimed design;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof with the phantom lines depicting environmental structure and forming no part of the claimed design.