xsl programming : increase parameter

* For loop from xsl files
<xsl:for-each select="/WSTResponse/Device">
<div id="detailDiv{@ID}">
ss
</div>
</xsl:for-each>

* Loop from style tag
<xsl:variable name = "a" >75</xsl:variable>

<xsl:for-each select="/WSTResponse/Device">
<xsl:variable name="rr"><xsl:number/></xsl:variable>
<xsl:if test="$rr = 1">
#detailDiv<xsl:value-of select="@ID"/> { right: 400px; top:<xsl:number value="75"/>px ; POSITION: absolute; }
</xsl:if>
<xsl:if test="$rr > 1">
#detailDiv<xsl:value-of select="@ID"/> { right: 400px; top:<xsl:number value="($rr*37)+38"/>px ; POSITION: absolute; }
</xsl:if>
</xsl:for-each>