Function is not working

Hi experts.This fucntion is not working. 
ssn is soscial security number.

CREATE OR REPLACE FUNCTION test.tel_to_apply (ssn varchar2(9))
   RETURN varchar2
IS
   myssn   varchar2(9);

BEGIN
   myssn := ssn

   IF myssn in (select * from test.aplicar)
   THEN
      myssn := to_char(to_number(ssn)+1);
   end if;
   RETURN myssn;
END;                                                             
/

RETURN myssn;
Error at line 13
ORA-00900: invalid SQL statement