How to sum of property over the ITEM Type

Hi Friends,

If i want to count Items on Item Type then i have a code.

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
 <xsl:output method="xml" omit-xml-declaration="yes"/>
 <xsl:template match="/"><Result><xsl:value-of select="count(//Item[@type='MACHINE ENTRY'])"/></Result></xsl:template>
</xsl:stylesheet>

Count is successful.

If I have a property name machine_hours and I want to sum machine_hours, so What changes required on above code.

Thanks in Advance

Abhishek Srivastava