Query Sqlite MIN Substr Charindex

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SQLite

i have the following data:

PRD-13
PRDF-4
ADQ-1
ASFS-77
AGXX-9
AMXX-14
AXX-17

I want to query the minimum that doesn't existe in following data, in this exemple it must be 2

I make one but it doesn't work

select MIN (substr(Code,charindex('_',Code)+1)+1) from produit where (substr(Code,charindex('_',Code)+1) + 1) not in (select (substr(Code,charindex('_',Code)+1)) from produit)