How to set Auto Incremented ID Field in XML file while inserting data

Hello Everyone 

there is 2 pages the first page include gridview its fill from xml file 

and there is button (Called add new student) when i click on this button its arrive me to second page .

and the second page inckude 5 Text box if to add new student.

** the problem with the ID of new student its can be show on the head page like this (http://localhost:57175/gridvieww/Default2.aspx?ID=8) i want head the ID number i am using query string 

>> now i want code to find the last node in xml and then increment +1 

this is my code 

XML code 

<Students>
  <Student>
    <ID>1</ID>
    <Name>hammad</Name>
    <Email>hammad@xml</Email>
    <Address>amman</Address>
    <Mobile>078889989</Mobile>
    <Class>fourth</Class>
  </Student>
</Students>