xpath simple sum

Hi  to everybody,

given,

<xsl:apply-templates
select="//Population_infos/Population_info[@Year='1900']"/>

I can show, for example, 

<xsl:template match="Population_infos/Population_info">
 <xsl:value-of select="@Year"/>
</xsl:template>

it works normally,

but when I want to sum all years of the selection range, it doesn´t work,
only shows years...

<xsl:template match="Population_infos/Population_info">
			<xsl:value-of select="sum(Population)"/>
</xsl:template>

any suggestion to sum them?...

Thanks

Juan

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