You must add a reference to assembly

Question

When I try to build my  project, I get following errors:

Error 1 :The type 'CultureInfo' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Globalization, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'

Error 2 :The type 'Task' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Threading.Tasks, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'

Error 3: The type 'Task<>' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Threading.Tasks, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'

 

I am working with Xamarin project. 

I am using visual Studio 2013 community and  add Xamarin in my visual Studio .

Can you please help me with this?