Help with a query, please

testdb=>
testdb=> select * from test1;

c|i
-+-
A|2
B|3
C|4
(3 rows)

testdb=> select * from test2;

c|i
-+-
A|6
C|7
(2 rows)

testdb=>

I'd like to get (test1.i + test2.i) like this:

c|sum
-+---
A|8
B|3
C|11

i.e. to sum the rows but if one of the rows doesn't exist on one of the
tables then print just the existing row. How can I do this?

Thanks in advance,
Timo T.