How to correctly put the "-" character in front of a variable in bat?

Question

cmd/bat

The code:

set f6=42 3t
robocopy -%f6% "R:\%f6%"

gives ERROR: Invalid parameter #1 : "-42"
I don't know where to put quotes and dashes to make it work
Without a dash it works, but you need it
With a full path
robocopy "E:\folder\-%f6%" "R:\%f6%"
it also works

Answer 1

Found a way out
Before the dash "-" it was necessary to put ".\"