arithmetic expressions in XSLT

Question

I need to multiply each of two nodes and find total sum, like quantity * perUnit and find total of all Products. How could it be done?

  <Products>
    <Product>
      <name>TestProduct</name>
      <quantity>3</quantity>
      <perUnit>14.88</perUnit>
    </Product>
    <Product>
      <name>Another Product</name>
      <quantity>1</quantity>
      <perUnit>2.45</perUnit>
    </Product>
    <Product>
      <name>One more product</name>
      <quantity>2</quantity>
      <perUnit>4.71</perUnit>
    </Product>
  </Products>

I tried to do it like this:

<xsl:variable name="TotalPrice">
    <xsl:for-each select="./Products/Product">
        <xsl:value-of select="quantity * perUnit"/>
    </xsl:for-each>
</xsl:variable>

And then

<xsl:value-of select="sum($TotalPrice)"/>

Also tried this:

<xsl:value-of select="sum(//quantity * //perUnit)"/>

But got an error:

Can not convert #NUMBER to a NodeList!