The ornamental design for a beam, as shown and described.
FIG. 1 is a perspective view of one end and two sides of a beam according to a first embodiment showing my new design, the opposite end and remaining two sides being identical to the end and sides as shown;
FIG. 2 is a perspective view of one end and two sides of a beam according to a second embodiment, the opposite end and two remaining sides being identical to the end and sides as shown;
FIG. 3 is a perspective view of one end and two sides of a beam according to a third embodiment, the opposite end and two remaining sides being identical to the end and sides as shown; and,
FIG. 4 is a perspective view of one end and two sides of a beam according to a fourth embodiment, the opposite end and two remaining sides being identical to the end and sides as shown.