<sections>
   <section start='3' end='8'>aaaa</section>
   <section start='10' end='12'>bbbb</section>
   <section start='14' end='22'>cccc</section>
</sections>

    
<xsl:template match="/">
        <xsl:variable name="num_seq" as="xs:double*">
            <xsl:for-each select="//subsection | //section">
                <xsl:sequence select="((@end)-(@start))"/>
            </xsl:for-each>
        </xsl:variable>
        <xsl:value-of select="sum($num_seq)"/>
</xsl:template>