This is my first time doing my work with HAP, but I find a problem with HtmlDocument class, I can't use it, because intellisense tells me that it is ambiguous reference, this is the picture : 

I have imported the library needed it this code which are using HTML_Agility;
using HtmlAgilityPack; and I also add reference to its DLL file but it can't help me so much. Do you know why this happens and how to fix it?
 
     
     
    