XSL 1.0 - sum values taken from corresponding column

Hello,

In <col2>, where cell = num, return the corresponding col1/cell/@value,
summing the result.

<root>
	<col1>
		<cell value="1">text</cell>
		<cell value="12">text</cell>
		<cell value="24">text</cell>
		<cell value="100">text</cell>
		<cell value="9">text</cell>
		<cell value="6">text</cell>
	</col1>
	<col2>
		<cell>num</cell>
		<cell>-</cell>
		<cell>num</cell>
		<cell>-</cell>
		<cell>num</cell>
		<cell>num</cell>
	</col2>
	<col3>
		<cell>-</cell>
		<cell>num</cell>
		<cell>num</cell>
		<cell>num</cell>
		<cell>num</cell>
		<cell>-</cell>
	</col3>
</root>

Sum for <col2> is: 40

Sum for <col3> is: 145

I'd like an XPath solution, but I don't think that is possible.

I tried using keys.

<xsl:key name="byValue" match="cell" use="/root/col1/cell[position() =
count(current()/preceding-sibling::cell)+1]/@value"/>

So each cell has the correct value, but I'm struggling to sum the values.
Maybe my thinking is wrong.

Any help appreciated.

--
Kevin