Macking Primary Key Like (dll-123456)

first to make column id like this(dll-123456) i made two column first is for three characters and second is int type and have a identity property but when combing two column i faced the problem of can not convert varchar to int so
i made it in one column which datatype is varchar and default value is (dll-1) to test at first after that using substring to have the number and add 1 to it but it is activated for 2 row only ,i can not konw why ?

ALTER TRIGGER Trigger2
ON dbo.Supplier
after INSERT--, UPDATE, DELETE */
AS
declare @supplierid varchar(100);
declare @sup varchar(20);
declare @nbr int;
select @nbr=nbr from inserted;

select @supplierid=supplierid from supplier where nbr=@nbr;
if @supplierid is null
select @supplierid='dll1';

select @sup=substring(@supplierid,4,1)+1;

if @sup <1
select @sup=1;
--select @last=substring(@supplierid,1,3);
insert into supplier(supplierid)
values
('dll'+@sup)
/* IF UPDATE () ...*/

so the column name is supplierid
so what the error or if there ara another way for this primary key please state it.
thank u in advance for assessting in that matter