How can we do operations with :root variables (css vars) in LESS?

Question

I´m trying to make operations with the root variables using less.

For example

:root{
  --padding-md: 5px;
}

@padding-md: ~'var(--padding-md)'; // we get the CSS var
@padding-double: @padding-md + 5px; //we try to add 5px

Or this

@padding-md: calc(5px + 3px) + 5px; // we get the CSS var

But LESS just throw the op Is this possible? How to do that?

there is a way to tell to LESS set the CSS CALC function instead make the operations without affect the default less operations?