The ornamental design for a pen, as shown and described.
FIG. 1 is a perspective view of a pen;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a perspective view wherein the pen clip on the top of the pen is pushed downward thereby enabling the pen point to be extended out of the pen for writing.
The broken line showing the pen point in FIG. 8 is included for the purpose of illustrating a portion of the pen that forms no part of the claimed design.