The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 2836CB0A8AC93F7A
Reading package lists... Done
W: GPG error: http://ppa.launchpad.net/kubuntu-ppa/backports/ubuntu artful InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 2836CB0A8AC93F7A
E: The repository 'http://ppa.launchpad.net/kubuntu-ppa/backports/ubuntu artful 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.