Automated Serial Number with strign

[egmv_appsheet]

I need to have customized Serial No. which is the key column, where this customized Sn is as follows

String-Value of A Column-count

For Example SN-AAA-0001

the next one SN-BXC-0002

SN is String
AAA, BXC, etc. value taken from the Column A
0001, 0002, etc Counter