The ornamental design for a calculator, as shown and described.
FIG. 1 is a perspective view from the top, left and front of a calculator.
FIG. 2 is a perspective view from the left, front and top of the calculator of FIG. 1.
FIG. 3 is a top plan view of the calculator of FIG. 1.
FIG. 4 is a rear end view of the calculator of FIG. 1.
FIG. 5 is a front end view of the calculator of FIG. 1.
FIG. 6 is a left side view of the calculator of FIG. 1; and,
FIG. 7 is a right side view of the calculator of FIG. 1.
The bottom of the calculator forms is no part of the claimed design and is not shown. The dashed lines showing the various parts of the calculator are for illustrative purposes only and form no part of the claimed design.