Task auto numbering

[ANGELSDIRTYFACE]

Hi all,Please see below, I need help with a formula or code for column A, I would like it to auto number depending on category chosen, For example if J2 was selected next the serial would be J2-003 because there are three J2 entries.
A B C D.
Serial J Category Task.
J1-001 J1 EXAMPLE EXAMPLE.
J4-001 J4.
J1-002 J1.
J1-003 J1.
J6-001 J6.
J1-004 J1.
J2-001 J2.
J2-002 J2.
J1-005 J1.
Any help would be great i cant seem to find anything anywhere.

 

[ANGELSDIRTYFACE]

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[Marcelo Branco]

Maybe

A2
=IF(B2<>"",B2&"-"&TEXT(COUNTIF($B$2:B2,B2),"000"),"")

copy down

 

[ANGELSDIRTYFACE]

Maybe

A2
=IF(B2<>"",B2&"-"&TEXT(COUNTIF($B$2:B2,B2),"000"),"")

copy down

Thanks for all your help that was so simple. xx

 

[Marcelo Branco]

Thanks for all your help that was so simple. xx

You are welcome and tks for the feedback

M.