How to sum a variable in XSL template?

Summary: I'm unable to a sum a variable data in XSL template.

Content (required):

Version (include the version you are using, if applicable): <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">

Code Snippet (add any code snippets that support your topic, if applicable):

we have a seeded report Disbursement payment file formats. as per the client requirement, we are designing a custom xsl template and we will add this custom template to above seeded report to get desired XML output. below is the sample output.

as per the requirement, we are able to calculate the total amount (<var_total>35</var_total>) for each payment based on hash total logic calculation (user defined) and storing it in a variable.