The following packages have unmet dependencies:
 ubuntu-desktop : Depends: gnome-system-monitor but it is not going to be installed
                  Depends: software-center but it is not going to be installed
                  Recommends: usb-creator-gtk but it is not going to be installed
                  Recommends: xul-ext-webaccounts but it is not going to be installed
E: Unable to correct problems, you have held broken packages.