create function syntax error

Hello

I Always get "syntax error" for every function that I try to create, for
example:
mydb'>create or replace function f_sum () returns void as '
mydb'> select 2+2;
mydb'> ' LANGUAGE plpgsql;
ERROR: syntax error at or near "select" at character 56
LINE 2: select 2+2;
^

What its the problem ??
Thank you,
Gustavo