How to add new child with value in existing XML file in C# using DOM

I have already an XML file whose structure is like this:

<?xml version="1.0" encoding="utf-16"?>
<configuration>
  <FilePath>C:\Recordings</FilePath>
  <Timer>2</Timer>
</configuration>

I want to to add one more child in <configuration> and want the new structure like shown below. Moreover, before adding I also want to make sure that the new child added is present or not.

<?xml version="1.0" encoding="utf-16"?>
<configuration>
  <FilePath>C:\Recordings</FilePath>
  <Timer>2</Timer>
  <LastSyncTime>Some Value</LastSyncTime>
</configuration>

If <LastSyncTime> is not there, then and only it should be added otherwise not.

I have tried so far as given below:

try
                {
                    XmlDocument doc = new XmlDocument();
                    doc.Load(newpath);
                    XmlNodeList nodes = doc.SelectNodes("LastSyncDateTime");


                    if(nodes.Count == 0)    //Means LastSyncDateTime node does not exist
                    {
                        XmlNode mynode = doc.CreateNode(XmlNodeType.Text, "LastSyncDateTime",null);
                        mynode.Value = DateTime.Now.ToString() + "";
                        doc.AppendChild(mynode);
                    }
                    foreach (XmlNode node in nodes)
                    {
                        if (node.LastChild != null)
                        {
                            LastSyncDateTime = (node.LastChild.InnerText);
                            Console.WriteLine("Last Date Time is : " + LastSyncDateTime);
                        }
                    }
                }
                catch (Exception ex)
                {
                    //throw ex;
                    string str = (String.Format("{0} {1}", ex.Message, ex.StackTrace));
                    Console.WriteLine(str);
                }

But I am getting exception. Please suggest me best working solution for this. Thanks