Excel xslt rowcount

I have an xml (for brevity sake):

<row>...</row>
<row>...</row>
<row>...</row>
<row>...</row>
<row>...</row>

the last "row" contains the totals, so I was trying to do something like this in the excel xslt (the number of "row" are dynamic)

<Cell ss:Formula="=SUM(R[-{count(row)}]C:R[-1]C)">
    <Data ss:Type="Number">
    <xsl:value-of select="." />
    </Data>
</Cell>

Because of circular reference problem, I need to get the count of row-1. I've tried:

ss:Formula="=SUM(R[-{count(row)-(count(row)-1)}]C:R[-1]C)"

ss:Formula="=SUM(R[-({count(row)})-(count(row)-1)]C:R[-1]C)"

but none of those are working. I know I could change the last "row" to "totals" and the row count would be right, but I'd like to do it w/o changing the xml.

Can someone help me with the syntax?

Thanks in advance!!!