Call functions from re.sub

This is a simple example:

import re

math='<m>3+5</m>'
print re.sub(r'<(.)>(\d+?)\+(\d+?)</\1>', int(r'\2') + int(r'\3'), math)

It gives me this error:

ValueError: invalid literal for int() with base 10: '\\2'

It sends \\2 instead of 3 and 5.

Why? How do I solve it?