How to do math functions

I was wondering how I can add or subtract a variable from a given number.
My script gets a number from a website using the "get text" option. I want to take that number and subtract it from a given value.
For example I tried making a variable, using the set variable option.
[[Number to use]] = 20  [[number extracted from website]]
However this doesn't work so could somebody point me in the right direction. ThanksEdit: I found the solution, just had to change the variable type to an "expression" instead of a string.

use expresion like [[number extracted from website]] > to digital
or JS fn