Installation problems

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Hi, i have ubuntu 12.04 installed few day ago. when i try to install ROS, my terminal give me this:

$ sudo apt-get install ros-indigo-desktop-full
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 ros-indigo-desktop-full : Depends: ros-indigo-desktop but it is not going to be installed
                           Depends: ros-indigo-perception but it is not going to be installed
                           Depends: ros-indigo-simulators but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

i'm not so good using ubuntu, but what can i do for solve the problem?? thank you very much!

Jacopo