how to update quantity in sqlserver.

how to update my quantity:

 

i get error when insert my quantity:

 

first time count 0 then insert but second time insert same product then get error how to get second time my quantity:like if first time insert then quantity one then we add same product then quantity 1+1 

create  PROCEDURE [dbo].[Sp_AddToMyCart]

@SessionId varchar(500),

@Product_Fkid int,

@Qty int

AS

Declare @count int,@ReturnValue int

BEGIN

Set @count=(Select COUNT(1) from dbo.MyCart where SessionId=@SessionId and Product_Fkid=@Product_Fkid)

if(@count=0)

begin

Insert into dbo.MyCart(SessionId,Product_Fkid,Qty) values (@SessionId,@Product_Fkid,@Qty)

Set @ReturnValue=1

end

else

begin

update dbo.MyCart

Set Qty=@Qty

where SessionId=@SessionId and Product_Fkid=@Product_Fkid

Set @ReturnValue=2

end

return @ReturnValue

END