Formulas in Google Docs Table( not spreadsheet)

i need to plug in the SUM formula, but in google docs instead of spreadsheets.i can't just use a calculator, i need to be able to plug in the formula like this:

SUM (20,35,90) and have it appear as 145 in the table cell.

does anyone know a google Add-On that would allow me to do this? i'd also accept the code BEHIND such an add-on.

(extensions aren't an option, as i need to do this on a work computer that disables them.)

Thanks in advance!