Calculate an average based on criteria. Exclude some rows.

I would like to calculate an average for a specific region excluding the current row.
For example, if I have the following table:

Store	Units Sold	Region
A	25	East
B	30	East
C	33	East
D	31	West
E	27	West
F	37	West

I would like to find that the "Remainder of Region Average" for store A is 31.5 (average of stores B and C)
Similarly, I would like to find that the "Remainder of Region Average" for store B is 29 (average of A and C)
etc...

I would want my results to look like this:

Store	Units Sold	Region	RemainAvg
A	25	East	31.5
B	30	East	29
C	33	East	27.5
D	31	West	32
E	27	West	34
F	37	West	29