[NTG-context] conversion from realpage to pagenumber

Hello list, how do I convert the realpage number to a page number? Suppose you have a document with a frontmatter of 20 pages in roman numerals, followed by the main text starting from page 1 in arabic numerals. So the twelfth page is 'XII', while the twenty-second is '2'. How do I get 'XII' from 12 and '2' from 22? Massi