We claim the ornamental design for a ladder, as shown and described.
FIG. 1 is a perspective view of a ladder showing our design;
FIG. 2 is a partial front view thereof;
FIG. 3 is a partial rear view thereof;
FIG. 4 is a partial top view thereof;
FIG. 5 is a partial bottom view thereof;
FIG. 6 is a partial side view thereof; and,
FIG. 7 is a partial side view thereof, being opposite the view illustrated in FIG. 6.
The ladder is shown in partial view in each of FIGS. 2-7 to more clearly illustrate the top portion of the claimed design.
Any broken lines and shading lines within these Figures showing the environment are for illustration purposes only and form no part of the claimed design.