> Maybe this is stupid question but how you can declare variable to be integer type
> Is below change the answer ??? ;-)

you are using xslt1 so there are no integers and you can not declare
types explictly, but writing
<xsl:variable name="p" select="number(./@p)"/>
or equivalently and more simply
<xsl:variable name="p" select="number(@p)"/>
will make the variable have type number (that is, double)

In XSLT2 you'll be able to add as="xs:integer" to make the typing
explict.

David