How do I convert string to number or vice versa?

When trying to call functions inside expressions, I often get "this parameter does not take a number" although the parameter is a single digit stored in a variable that is parsed from string (removed surrounding characters). There is also no "any" parameter type on functions. I need to convert numbers into strings and back, then do maths with those numbers.

construct.net/en/make-games/manuals/construct-3/system-reference/system-expressions

str()

int()

float()