XPath help for distinct values

I am trying to pull distinct values from an XML file. This is what I have but doesn’t work.

How can I pull distinct @City values?

XPATH (Doesn’t get distinct @City values)


//Cty[not(@City=preceding-sibling::Cty/@City)]/@City

XML SAMPLE


<documentRoot>
&#65279;<?version="1.0" encoding="utf-8"?>
<Exp800 Promo="InsiderSelect_Regions" >
  <SuperReg SuperRegion="Caribbean" >
    <Cntry Country="Aruba" >
      <Cty City="Palm Beach" ></Cty>
    </Cntry>
  </SuperReg>
  <SuperReg SuperRegion="Mexico and Central America" >
    <Cntry Country="Panama" >
      <Cty City="Panama City" ></Cty>
    </Cntry>
  </SuperReg>
  <SuperReg SuperRegion="United States of America" >
    <Cntry Country="Florida" >
      <Cty City="Palm Beach" ></Cty>
      <Cty City="Panama City" ></Cty>
    </Cntry>
  </SuperReg>
</Exp800>
</documentRoot>

Try the following XPATH:
//SuperReg/Cntry/Cty[not(@City=preceding::SuperReg/Cntry/Cty/@City)]