Subtracting in XSL?

I have an XML document that contains the following:
<DOCUMENT>
	<Image_A><IMG SRC="image_a.jpg" WIDTH="200" HEIGHT="400"/></Image_A>
	<Image_B><IMG SRC="image_b.jpg" WIDTH="100" HEIGHT="200"/></Image_B>
</DOCUMENT>

Is there a way for me to subtract the height of the "Image_B" element from
the "Image_A" element?

The way I'm doing it right now involves a VBScript function I created in my
XSL:
<msxsl:script language="VBScript" implements-prefix="test">
  Function Subtract(n1, n2)	
	Subtract = n1 - n2 
  End Function
</msxsl:script>

I call the function like this:
<xsl:value-of select="test:Subtract(<xsl:value-of
select="//DOCUMENT/Image_A/IMG/@HEIGHT">, <xsl:value-of
select="//DOCUMENT/Image_B/IMG/@HEIGHT">)" />

This, obviously, doesn't work, but is there a way to pass the HEIGHT values
into my Subtract function?? If not, is there a better way of accomplishing
this?

Thanks in advance!


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