how to sum tow field of table then insert into next row ?

hello,

Please check this code. may this help you. if any problem plz reply.
// For create a table
CREATE TABLE [tbl_DemoCsharp](
  [Id] [int] IDENTITY(1,1) NOT NULL,
  [SumV] [int] NOT NULL,
  [AddValue] [int] NULL,
 CONSTRAINT [PK_tbl_DemoCsharp] PRIMARY KEY CLUSTERED
(
  [Id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

and

this is a query
//i think Addvalue is a value that provide by you.

Update tbl_DemoCsharp
Set Addvalue=200
where id= (Select top 1 id from tbl_DemoCsharp Order by id Desc)

Insert into tbl_DemoCsharp(sumv)
Select top 1 Sumv + 200 as Sumval from tbl_DemoCsharp Order by id Desc


Select * from tbl_DemoCsharp