asp.net 中如何得到GridView 中某一列的值的和

如题  下面是代码 可以按月份查找出来，什么把其中一列的值的总和求出来最后面的那个按钮还空着请各位大虾帮忙看看
using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.OleDb;
using System.Data.Common;
using System.Text;

public partial class admin_SelectIntoSingle : System.Web.UI.Page
{


    protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {

            MyDataBind();
        }

    }
    public void MyDataBind()
    {
        OleDbConnection MyConnection = new OleDbConnection(@"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + Server.MapPath("access.mdb"));
        OleDbDataAdapter MyCommand = new OleDbDataAdapter(@"SELECT * from [Into Single] ", MyConnection);
        DataSet ds = new DataSet();
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