Varchar to integer on the fly in Postgres

gistfile1.sql

db=> CREATE FUNCTION toint(varchar) 
RETURNS integer 
STRICT IMMUTABLE LANGUAGE SQL AS 
'SELECT cast($1 as integer);';


db=> \d public.items
        Column         |          Type
-----------------------+------------------------
...
 description           | character varying(255)
...
 lastvalue             | character varying(255)
...


db=> select sum(lastvalue) from public.items where description like '%YOUR ITEM%' and lastvalue is not null;

ERROR: function sum(character varying) does not exist
LINE 1: select sum(lastvalue) from public.items where description li...


db=> select sum(toint(lastvalue)) from public.items where description like '%YOUR ITEM%' and lastvalue is not null;

sum
-------
100000
(1 row)