Increment Two column in a single table in sql server

CREATE TABLE myTableName
(
 GRNo INT IDENTITY(1,1) NOT NULL,
 SecondColumnName VARCHAR(100),
 OtherColumnsName VARCHAR(100),
 BillNo AS ( 'B' + CAST( GRNo AS varchar(max) ) ) -- filling the value by computing B1 if GRNo is 1
-- OR Simply
-- BillNo AS (GRNo ) -- filling with GRNo
)