i am trying to installing rolling ridley on Ubuntu 20.04 but getting error :Unable to locate package ros-rolling-desktop

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Hi , i am

Err:5 http://packages.ros.org/ros2/ubuntu focal InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F42ED6FBAB17C654
Reading package lists... Done
W: GPG error: http://packages.ros.org/ros2/ubuntu focal InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F42ED6FBAB17C654
E: The repository 'http://packages.ros.org/ros2/ubuntu focal InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
s@V:~$ sudo apt install ros-rolling-desktop
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package ros-rolling-desktop
s@V:~$ sudo apt install ros-rolling-ros-base
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package ros-rolling-ros-base