How to get attribute value from node using xpath?

Totally new to xpath.

I have xml:

<Values>
   <Value Value="a" CustomAtr="1" />
   <Value Value="b" CustomAtr="2" />
   <Value Value="c" CustomAtr="3" />
</Values>

To get value by index, i can use: string(/*/Value[1]/@CustomAtr)

Is it possible to get value by key? Hot to get CustomAtr value where Value="b" ?

Thank you