Execute Function in Stored Procedure SQL Server

--CREATE A FUNCTION FOR ADD TWO USER INPUT VALUES.
CREATE FUNCTION AddFun_sp(@NumIst INT, @NumpIInd INT)
RETURNS INT
AS BEGIN
DECLARE @Result as int
    SET @Result=@NumIst+@NumpIInd
RETURN @Result
END

--CALL A FUNCTION IN STORE PROCEDURE.
CREATE PROCEDURE usp_CallAfunction
(
 @IstVar INT,
 @IIndVar INT
)
AS BEGIN
DECLARE @sp AS INT
    SET @sp=dbo.AddFun1(@IstVar,@IIndVar)
PRINT @sp
END

--EXECUTE STORE PROCEDURE FOR OUTPUT RESULT.
EXECUTE dbo.usp_CallAfunction '10', '20'

--RESULT IS 30