The ornamental design for a calculator, as shown and described.
FIG. 1 is a top plan view thereof;
FIG. 2 is a right side elevation view thereof;
FIG. 3 is a rear elevation view thereof; and,
FIG. 4 is a front elevation view thereof.
The broken line illustrations on the keys in the drawing figures are for illustrative purposes only and form no part of the claimed design. The contrast in shading techniques in FIGS. 1–4 represents a color contrast in appearance of these elements and does not represent any specific color, material, or texture.