auto_increment code stored procedures sql server

Question

this code work but auto_increment not work what is the right code.. correct me

 

ALTER PROCEDURE [dbo].[GRNInfo]

@GrnCode nchar(10) ,

@ItemCode nchar(10) ,

@Qty int ,

@TotalPrice decimal(18, 0) ,

@GrnDate date

AS

BEGIN

DECLARE @Max INT

if not exists (select GrnCode from atoGRN where GrnCode=@GrnCode)

begin

 ------------------------------------------------------------------------------------------------------------------

--SET @GrnCode = 'GRN00001'

SELECT @Max = CONVERT(INT, SUBSTRING(CONVERT(VARCHAR(10),@GrnCode), 3, 10)++1) FROM atoGRN

SET @GrnCode = 'GRN' + RIGHT('0000' + CONVERT(VARCHAR(10),@Max ++ 1 ) , 5)

 

---------------------------------------------------------------------------------------------------------- 

insert into atoGRN (GrnCode,FK_ItemCode,Qty,TotalPrice,GrnDate) values (@GrnCode,@ItemCode,@Qty,@TotalPrice,@GrnDate)

end