Multiply number with column value in SQL Server

hi

in film.aspx is textbox that users can enter number I want this number multiply with Pricedvd column's data in database:

i.e

if users enter 5 in textbox I want table change like:

below are code:

    protected void Zarib_Click(object sender, ImageClickEventArgs e)
    {

        using (SqlConnection conn = General.GetConnection())
        {
            using (SqlCommand _cmd = General.GetCommand("Zarib_Update", conn))
            {
                conn.Open();
                _cmd.Parameters.AddWithValue("@Zarib", int.Parse(Txtzarib.Text));
                _cmd.ExecuteNonQuery();
                Response.Redirect("Film.aspx");
            }
        }
    }

SP:

ALTER procedure [dbo].[Zarib_Update]
@Zarib varchar(5)
AS
BEGIN
declare @PriceDvd Int SET @PriceDvd=(SELECT PriceDvd From SerialFilm where daste='فیلم')
UPDATE SerialFilm
SET
PriceDvd=@Zarib*@PriceDvd
END

but here when I enter number in text box it insert NULL in Pricedvd column:

can you help me?

Best Regards

Neda