In your Ubuntu Desktop page, goto (in left vertical bar of icons):
Ubuntu Software Center -> Edit -> Software Sources -> (Here Select in Download from:) Server for United States.
After doing this and saving, follow the installation procedure given here:
http://wiki.ros.org/hydro/Installatio...
This should install without any errors now. The reason why your ROS didn't install initially may be due to 'Software Sources' which did not contain updated repository for Hydro. I hope this works now. Best Wishes!
PS: It is not necessary that you only select Server for United States all the times. It depends on which server the complete package is available. Right now, it is available at Server for United States.
Please edit your question to include the exact command and copy and paste the error message.
you probably try to install ros-indigo-desktop-full on 12.04 which doesnt exist?
yes mehdi when i go to software centre it said that it does not exist. but i install it successfully on 14.04 but my professor insist on 12.04 i study all in ROS web site about how to install it but i does not work and i think the problem is not from my side.