question about add dll in unity

I try to add dll in unity, but always have error:dllnotfoundexception. I release dll in x64 and my unity is 64 bit too, and I already put my dll in folder "Plugins" here is my dll code: ![alt text][1] and in unity I do ![alt text][2] I think the dll I creat it is quite simple, but why I can not use it? [1]: /storage/temp/114636-1.jpg [2]: /storage/temp/114640-3.jpg