list of missing numbers within a defined range

Good day
I have a "consecutive" table where I place a range of [start] and another of [end], to restrict the numbers that users place in a column type number [Label] of another "Registration" table.

I would like to generate a list that tells me the numbers that have not been used within the assigned range, only if they have a greater consecutive, example:
If they are assigned a start-to-end range 1-20, and the user has used:
1,2,3,6,7,9
tell me that the numbers "4,5,8" are missing, or failing that "4,5,8,10,11,12,13....20")

I appreciate your help.

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