The ornamental design for a cartonblank, as shown and described.
FIG. 1 is a top plan view of a carton blank showing my new design, the
bottom plan view being the same in appearance;
FIG. 2 is a left side elevation thereof, the right side being the same in
appearance;
FIG. 3 is a front elevation thereof; and,
FIG. 4 is a rear elevation thereof.
The solid lines in the top plan view represent micro cuts, and the dashed
lines represent fold lines, in the blank structure the zig-zag line
represents tear line.