apt-get upgrade дает GPG Error [duplicate]

Как мне исправить эту ошибку? У меня установлен pycharm через ppa, который я использую уже много лет на разных компьютерах.

W: GPG error: http://ppa.launchpad.net/mystic-mirage/pycharm/ubuntu xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY DD969F10A7E2BCD2
W: The repository 'http://ppa.launchpad.net/mystic-mirage/pycharm/ubuntu xenial InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: There is no public key available for the following key IDs:
DD969F10A7E2BCD2