how to sum the grand total of the product of two value?

I have a xml file and a xsl file as show in the following, how can i sum the total of the product of the values of "amount" and "cost" ? what wrong with in following xsl, the output is messy .......


<!-- lineItems.xml -->
<?xml version='1.0' encoding="ISO8859-1" ?>
<?xml-stylesheet type="text/xsl" href="lineItems.xsl" ?>
<lineItems>
<lineItem>
<code>42AC5</code>
<title>Loopy Fruit Cereal</title>
<amount>12</amount>
<cost>4.25</cost>
</lineItem>
<lineItem>
<code>H343A</code>
<title>MicroSecond Rice</title>
<amount>14</amount>
<cost>2.35</cost>
</lineItem>
<lineItem>
<code>EA198</code>
<title>Crescent Toothpaste</title>
<amount>18</amount>
<cost>1.95</cost>
</lineItem>
</lineItems>



<!-- lineItems.xsl -->
<?xml version='1.0' encoding="ISO8859-1"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

<xsl:template match="/">
<xsl:variable name="newLineItems">
<xsl:for-each select="/lineItems/lineItem">
<lineItem>
<xsl:copy-of select="*"/>
<subTotal><xsl:value-of select="amount*cost"/></subTotal>
</lineItem>
</xsl:for-each>
</xsl:variable>
<xsl:variable name="lineItemsTotal">
<xsl:value-of select="sum($newLineItems/subTotal)"/>
</xsl:variable>
<xsl:value-of select="$lineItemsTotal"/>
</xsl:template>
</xsl:stylesheet>



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