I try to install ROS 2 on ubuntu focal fosa (20.04).

[I went along with this instruction](https://docs.ros.org/en/foxy/Installation/Ubuntu-Install-Debians.html), but
after implementation this command:

```
sudo apt update

```

I got this problem:

```
Hit:1 http://ru.archive.ubuntu.com/ubuntu focal InRelease
Hit:2 http://ru.archive.ubuntu.com/ubuntu focal-updates InRelease                                                                                       
Hit:3 http://ru.archive.ubuntu.com/ubuntu focal-backports InRelease                                                                                                   
Hit:4 http://download.opensuse.org/repositories/home:/Alexx2000/xUbuntu_20.04  InRelease                                                                              
Hit:5 http://security.ubuntu.com/ubuntu focal-security InRelease                                                                                                      
Hit:6 http://packages.osrfoundation.org/gazebo/ubuntu-stable focal InRelease                                       
Get:7 http://packages.ros.org/ros2/ubuntu focal InRelease [4,670 B]
Err:7 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.

```

And after that: `sudo apt install ros-foxy-desktop`

I got it:

```
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package ros-foxy-desktop

```