sum of attribute values

Hi,
I need a sum of all the @width attributes from the following xml. I was using
<xsl:value-of select="sum(number(substring-before(//@COLWIDTH, 'in')))"/>. But
it did not work. Can anyone help me with this.

<table>
	<cols>
		<col colnum="1" width="0.984in"/>
		<col colnum="2" width="1.880in"/>
		<col colnum="3" width="0.984in"/>
	</cols>
</table>

Thanks,
Navpreet.