Visual Studio 2013 problem during develop basic SharePoint-hosted app

Hi,

i am begginer in developing Sharepoint apps using Visual studio 2013.

I followed this instructions http://msdn.microsoft.com/en-us/library/fp142379.aspx for creating a basic SharePoint-hosted app.  

When i tried to build my application (Step 10 in create and deploy your first Sharepoint-hosted app) i got the following error: 

What is the problem?

thanks, Aphrodite