I claim the ornamental design for a calculator, as shown and described.
FIG. 1 is a perspective view of a calculator, showing my new design;
FIG. 2 is a front elevational view of the invention;
FIG. 3 is a rear elevational view of the invention;
FIG. 4 is a left-side elevational view of the invention;
FIG. 5 is a right-side elevational view of the invention;
FIG. 6 is a top plan view of the invention; and,
FIG. 7 is a bottom plan view of the invention.
The symbol indicia illustrated in broken lines on the round keys in FIGS. 1 and 6 are illustrative only, and form no part of the claimed design. The thick broken line in FIG. 7 defines the bounds of the claimed design, within which is illustrated environmental matter in thin broken lines that forms no part of the claimed design.