SQL UPDATE on with data from same table

I have a next situation with a table, Need to update rows from rows in same table, as you see here:

TABLE

ID	SN	FID
1	12345	1
2	1122	2
3	12345-RG	NULL
4	1122-RG	NULL

I need to UPDATE row 3 from row 1 Column FID(Same SN, different ending only on the end -RG) Update row 4 FROM row 2 Column FID(Same SN, different ending -RG)

So the result should be:

TABLE

ID	SN	FID
1	12345	1
2	1122	2
3	12345-RG	1
4	1122-RG	2

I have tried many ways but I don’t get with this… I tried declaring a temp table and trying to compare from there but still with this issue…

Answer

Asuming [SN] is being augmented with -RG

Example

with cte as (
Select * 
      ,NV = max(FID) over (partition by replace(SN,'-RG','') )
 From YourTable
)
Update cte set FID = NV 
 --Where FID is null  -- Optional

The Updated Table

ID  SN          FID
1   12345       1
2   1122        2
3   12345-RG    1
4   1122-RG     2