Error while installing ROS on ubuntu 12.04

When i run the

sudo apt-get install ros-hydro-desktop-full


i get this error message 

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

Solution :

sudo aptitude install ros-hydro-desktop-full