How do i get a unique sequence no in a formula in salesforce

I am trying to have a formula field which has a unique seq no at th开发者_Python百科e end of the formula

 "PJ_"&SHORTNAME&"_"&UNIQUE-SEQ-NUM

Any idea how we can achieve this

Thanks

You need to define a field on the object of type "AutoNumber". You can then either use the display format property of it to achieve what you want or alternatively define a separate formula field (as per your example) but reference the AutoNumber field in place of UNIQUE-SEQ-NUM