Append to XML file content and sum the number of nodes in the XML

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C#

XML

After I click the button, the file is generated, the xml file will update the nodes with those files created in the existing xml file and sum up all the nodes in the records.

How do I append the node into existing xml file?

What I have tried:

Content of xml file:

<?xml version="1.0" encoding="utf-8"?>
<root>
	<rec>
		<totalrec>3
	
	<datas>
		<data>
			<id>000
			<file>20170111000U.TIF
			<file>20170111000R.TIF
		</data>
		<data>
			<id>001
			<file>20170111001U.TIF
			<file>20170111001R.TIF
		</data>
		<data>
			<id>002
			<file>20170111002U.TIF
			<file>20170111002R.TIF
		</data>