get total page

Hi,

I was trying to get the total page like 2/5 but I always met this message  "ref id not found". I made something like the following : 
 <fo:flow>
           <fo:block id="last-page"/> 
            
  </fo:flow>
...........................
<fo:page-number/>    
<fo:page-number-citation ref-id="last-page"/>

is there someone who can help me?

Best regards,
Rija