FIG. 1 is a front elevation view of a calculator, showing our new design; FIG. 2 is a top plan view thereof; FIG. 3 is a bottom plan view thereof; FIG. 4 is a left side elevation view thereof; FIG. 5 is a right side elevation view thereof; FIG. 6 is a back elevation view thereof; and, FIG. 7 is a perspective view thereof. The broken lines depict environmental subject matter only and form no part of the claimed design.