PL/SQL :: Select Record Between Date Gap

I have a sql query where I need to select only records with an 18 month gap between max(date) and previous date( no dates between max(date)and 18 month gap date), when I run the below query it should only select supid 130, not 120 (even though 120 does contain an 18 month gap date it also has a date that is less then the 18 month gap( '25-NOV-2012','DD-MON-YYYY'). how would get the query to look back 18 months for the next date and evaluate the month_between.

. example: 

create table supply(supID number(8), supply varchar2(20), supdate Date,supamount number(13,2));
 insert into supply values(100,'Tapes',to_date('01-AUG-2013','DD-MON-YYYY'),50.00);
insert into supply values(100,'TV',to_date('01-APR-2013','DD-MON-YYYY'),250.00);
insert into supply values(100,'Discs',to_date('25-DEC-2012','DD-MON-YYYY'),25.00);
insert into supply values(120,'Tablets',to_date('25-AUG-2013','DD-MON-YYYY'),15.00);

[Code]....

and p.supid in(select s.supid from supply s where months_between

(s.supdate,p.supdate)<-18)      SUPID SUPPLY               SUPDATE    SUPAMOUNT---------- -------------------- --------- ----------       120 Tablets              25-AUG-13         15       130 Discs                25-JUL-13         75