How to auto increment id in new nodes?

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C#

ASP.NET

//////By this code i am getting output as:

<?xml version="1.0" encoding="UTF-8"?>
<Album>
  <ImagesFolder>
    <Image Id="1" Name="Image0356.jpg" ImagePath="D:\OrgWebsite\Traditional Day 2011\Images\Image0356.jpg" />
    <Image Id="2" Name="4.jpg" ImagePath="D:\OrgWebsite\Traditional Day 2011\Images\4.jpg" />
    <Image Id="2" Name="Shilpa1.jpg" ImagePath="D:\OrgWebsite\Traditional Day 2011\Images\Shilpa1.jpg" />
  </ImagesFolder>
  <VideosFolder>
    <Video />
  </VideosFolder>

</Album>

/////// this is my code////

public void createxml()
  {
      XmlDocument xmldoc = new XmlDocument();
      XmlNode ImagesFolder = xmldoc.CreateElement("ImagesFolder");
      XmlNode VideosFolder = xmldoc.CreateElement("VideosFolder");
      XmlNode Image = xmldoc.CreateElement("Image");
      XmlNode Video = xmldoc.CreateElement("Video");
      XmlNode albumnode = xmldoc.CreateElement("Album");
      try
      {
          if (xmlFolder.Exists)
          {

              DirectoryInfo di = new DirectoryInfo(Server.MapPath("") + "\\" + txtalbum.Text + "\\" + "XML\\");
              FileInfo[] TXTFiles = di.GetFiles("*.xml");
              if (TXTFiles.Length == 0)
              {

                  XmlNode docNode = xmldoc.CreateXmlDeclaration("1.0", "UTF-8", null);
                  xmldoc.AppendChild(docNode);
                  XNamespace adlcp = "http://www.w3.org/2001/XMLSchema-instance";

                  xmldoc.AppendChild(albumnode);
                  //XmlNode ImagesFolder = xmldoc.CreateElement("ImagesFolder");
                  //XmlNode VideosFolder = xmldoc.CreateElement("VideosFolder");
                  //XmlNode Image = xmldoc.CreateElement("Image");
                  //XmlNode Video = xmldoc.CreateElement("Video");
                  XmlAttribute Imgattribute1 = xmldoc.CreateAttribute("Id");
                  Imgattribute1.Value = "1";
                  Image.Attributes.Append(Imgattribute1);

                  XmlAttribute Imgattribute2 = xmldoc.CreateAttribute("Name");
                  Imgattribute2.Value = FileUpload1.FileName;
                  Image.Attributes.Append(Imgattribute2);

                  XmlAttribute Imgattribute3 = xmldoc.CreateAttribute("ImagePath");
                  Imgattribute3.Value = "" + location;
                  Image.Attributes.Append(Imgattribute3);

                  albumnode.AppendChild(ImagesFolder);
                  albumnode.AppendChild(VideosFolder);
                  ImagesFolder.AppendChild(Image);
                  VideosFolder.AppendChild(Video);
                  xmldoc.Save(Server.MapPath("" + "~\\" + txtalbum.Text + "\\XML\\" + txtalbum.Text + ".xml"));

              }
              else
              {
                  //updating the nodes

                  xmldoc.Load(Server.MapPath("" + "~\\" + txtalbum.Text + "\\XML\\" + txtalbum.Text + ".xml"));
                  int flgnode = 0;
                  XmlNodeList nodelist = xmldoc.GetElementsByTagName("Image");
                  for (int i = 0; i < nodelist.Count; i++)
                  {
                      string getnode = nodelist[i].Attributes["Name"].Value;
                      if (FileUpload1.FileName.Equals(getnode))
                      {
                          flgnode = 1;
                          Response.Write(FileUpload1.FileName + " Is already Present. Please Choose Another");
                      }
                  }
                  if (flgnode == 0)
                  {

                      XmlNode newimg = xmldoc.CreateElement("Image");

                      XmlAttribute Imgatt1 = xmldoc.CreateAttribute("Id");
                      Imgatt1.Value = "2";
                      newimg.Attributes.Append(Imgatt1);

                      XmlAttribute Imgatt2 = xmldoc.CreateAttribute("Name");
                      Imgatt2.Value = FileUpload1.FileName;
                      newimg.Attributes.Append(Imgatt2);

                      XmlAttribute Imgatt3 = xmldoc.CreateAttribute("ImagePath");
                      Imgatt3.Value = "" + location;
                      newimg.Attributes.Append(Imgatt3);

                      xmldoc.DocumentElement.AppendChild(newimg);

                      XmlNodeList lstxmlNode = xmldoc.GetElementsByTagName("ImagesFolder");
                      lstxmlNode[0].AppendChild(newimg);


                      xmldoc.Save(Server.MapPath("~/"+txtalbum.Text+"/XML/Traditional Day 2011.xml"));
                      Response.Write("Image Add Successfully...");

                  }
              }
          }
      }
      catch (Exception ex)
      {
          throw ex;
      }
  }

/// i want output as/////

<?xml version="1.0" encoding="UTF-8"?>
<Album>
  <ImagesFolder>
    <Image Id="1" Name="Image0356.jpg" ImagePath="D:\OrgWebsite\Traditional Day 2011\Images\Image0356.jpg" />
    <Image Id="2" Name="4.jpg" ImagePath="D:\OrgWebsite\Traditional Day 2011\Images\4.jpg" />
    <Image Id="3" Name="Shilpa1.jpg" ImagePath="D:\OrgWebsite\Traditional Day 2011\Images\Shilpa1.jpg" />
  </ImagesFolder>
  <VideosFolder>
    <Video />
  </VideosFolder>

</Album>

// how can i do this..