How to sum a sequence of numbers in XML?

Question

XML

You need to write a transformation for an xml document that considers the sum of a sequence of numbers specified in the document in the from and to elements

Document example:

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="some.xsl"?>
<sum-of-sequence>
<from>1</from>
<to>10</to>
</sum-of-sequence>