Calculate column total in GridView footer using C# in ASP.Net

hello,

i would like calculate total point from user the user has 76 point

Am done with total each record so. When i search for user all his point i get total and i want calculate ((total point - 76 Point) = am looking how much left point he has

check the screenshoot

code c#

        private void BindGrid()
        {
            string constr = ConfigurationManager.ConnectionStrings["kankonConnectionString"].ConnectionString;
            using (SqlConnection con = new SqlConnection(constr))
            {
                using (SqlCommand cmd = new SqlCommand("vacation"))
                {
                    cmd.Parameters.AddWithValue("@Action", "SELECT");
                    using (SqlDataAdapter sda = new SqlDataAdapter())
                    {
                        cmd.CommandType = CommandType.StoredProcedure;
                        cmd.Connection = con;
                        sda.SelectCommand = cmd;
                        using (DataTable dt = new DataTable())
                        {
                            sda.Fill(dt);
                            GridView1.DataSource = dt;
                            GridView1.DataBind();
                            GridView1.FooterRow.Cells[11].Text = "الرصيد المستخدك";
                            GridView1.FooterRow.Cells[12].Text = dt.Compute("Sum(totalva)", "").ToString();
                            GridView1.FooterRow.Cells[13].Text = "الرصيد المتبقي";                           
                        }
                    }
                }
            }
        }