Passing parameter into tag name

Hello all,
I have a data scenario like this (edited down for clarity) where I have
multiple tags that have the actual level number in the tag name:
<foo> <Level> <Level_No>1</Level_No> </Level> <Level> <Level_No>2</Level_No>
</Level> <Level> <Level_No>3</Level_No> </Level> <Cost ID='1'>
<Level_1_Amt>100</Level_1_Amt> <Level_2_Amt>200</Level_2_Amt>
<Level_3_Amt>300</Level_3_Amt> </Cost> <CostB ID='2'>
<Level_1_Amt>50</Level_1_Amt> <Level_2_Amt>100</Level_2_Amt>
<Level_3_Amt>150</Level_3_Amt> </Cost> <CostB ID='3'>
<Level_1_Amt>175</Level_1_Amt> </Cost> <CostB ID='4'>
<Level_3_Amt>400</Level_3_Amt> </Cost></foo>
I am trying to sum up the amounts in the following code, but am getting a
syntactical error about how I am trying to pass the parameter value into the
tag name:
<xsl:stylesheet version="2.0" xmlns="http://www.w3.org/1999/xhtml"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xs="http://www.w3.org/2001/XMLSchema"> <xsl:output method="xml"
indent="yes"/>
 <xsl:template name="buildLevel"> <xsl:param name="levelNum"/> <Level_Amt>
<xsl:value-of select="sum(/Story/Level_{$levelNum}_Amt)"/> </Level_Amt>
</xsl:template>

 <xsl:template match="Cost"> <xsl:for-each select="/foo/Level"> <Level>
<xsl:call-template name="buildLevel"> <xsl:with-param name="levelNum"
select="Level_No"/> </xsl:call-template> </Level> </xsl:for-each>
</xsl:template> </xsl:stylesheet>
Is there a way I can do something like this? I am trying to avoid explicitly
calling each tag by name since there could be up to 9 levels in the data and
there are multiple tags that have the level number in them. Thank you in
advance.B