Want to show value of variable in alert box

public partial class WebForm1 : System.Web.UI.Page
    {
        int sum3;

        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            { 
                DataTable dTable = new DataTable("mainTable");

                DataColumn Name = new DataColumn("Name");
                dTable.Columns.Add(Name);

                DataColumn Marks = new DataColumn("Marks");
                dTable.Columns.Add(Marks);

                DataColumn Marks2 = new DataColumn("Marks2");
                dTable.Columns.Add(Marks2);

                DataColumn Total = new DataColumn("Total");
                dTable.Columns.Add(Total);

                int sum = 0;
                int sum2 = 0;

                DataRow dRow = null;
                for (int i = 0; i < 5; i++)
                {
                    dRow = dTable.NewRow();
                    dRow["Name"] = i + " - Sagar";
                    dRow["Marks"] = i;
                    sum += i;
                    dRow["Marks2"] = i + 5;
                    sum2 += i + 5;
                    dRow["Total"] = i + i + 5;
                    sum3 += i + i + 5;
                    dTable.Rows.Add(dRow);
                }

                Page.ClientScript.RegisterStartupScript(this.GetType(), "alert", "showAlert(" + sum3.ToString() + ");", true);

                //dRow = dTable.NewRow();
                //dRow["Name"] = "Total";
                //dRow["Marks"] = sum;
                //dRow["Marks2"] = sum2;
                //dRow["Total"] = sum3;
                //dTable.Rows.Add(dRow);

                aspDataGrid.DataSource = dTable;
                aspDataGrid.DataBind();
            }
        }

        protected void btn_Submit_Click(object sender, EventArgs e)
        {
            marks1.Text = "Total Marks : " + sum3.ToString();
            Page.ClientScript.RegisterStartupScript(this.GetType(), "alert", "showAlert(" + sum3.ToString() + ");", true);
        }
}