我最近升级到Ubuntu 12.04LTS并安装了Tomcat 7，但出现以下错误：
root@me:/# sudo apt-get install tomcat7 tomcat7-docs tomcat7-examples tomcat7-admin -y
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:
tomcat7 : Depends: tomcat7-common (>= 7.0.26-1ubuntu1.1) but it is not going to be installed
tomcat7-admin : Depends: tomcat7-common (>= 7.0.26-1ubuntu1.1) but it is not going to be installed
tomcat7-docs : Depends: tomcat7-common (>= 7.0.26-1ubuntu1.1) but it is not going to be installed
tomcat7-examples : Depends: tomcat7-common (>= 7.0.26-1ubuntu1.1) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
请帮我解决这个问题。