It's a tiny calculator icon in the status bar. Click it, to expand it and use it. Click the '+' to change to '-', '/', or '*'. Click '=' to get the answer (output to the first textbox). Or, if you don't like to use the mouse, you can use the +, -, *, /, ^, =, and ENTER function keys to do the calculations instead (just like a calculator). Click the calculator icon to shrink it to just the icon when not using it. Ctrl+Alt+C to expand/collapse and set focus, Ctrl+Alt+Numpad4(or tab) to set focus from second textbox back to first text box and empty both boxes, Shift+Tab to set focus from second text box to first text box and highlight contents of first textbox