How to write a WCF Service for SharePoint 2013 hosted App?

I’ve deployed a WCF Service in SharePoint. I am able to retrieve data from it using URL:

http://nit-hv8-itr4:1111/_vti_bin/PresidentsService.svc/GetAllPresidents

The problem occurs when I try to access this same service through REST from SharePoint hosted App saying – PresidentsService.svc is not accessible from the context of a SharePoint App.

I also tried changing the URL of Service as follows:

http://apps-a6d8c7c05a0ab7.spappsitr4.com:1111/_vti_bin/PresidentsService.svc/GetAllPresidents

Here I get error saying – The endpoint /_vti_bin/presidentsservice.svc is not accessible in the context of a SharePoint App.