installation of gazebo turtlebot simulator

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Hi all, I apologise in advance I'm totally new to this. I'm trying to install turtlebot with gazebo simulator.

I followed the instructions here: http://wiki.ros.org/turtlebot_gazebo/...

I typed

sudo apt-get install ros-indigo-turtlebot-apps ros-indigo-turtlebot-rviz-launchers

into the terminal but received these errors.

Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package ros-indigo-turtlebot-apps
E: Unable to locate package ros-indigo-turtlebot-rviz-launchers

How do I get the packages? Thanks a lot in advance.