APT - Error GPG Key

When updating repository and get the error below, add the key based on last 8 characters:

Reading package lists... Done
W: GPG error: http://deb.opera.com stable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F9A2F76A9D1A0061
W: You may want to run apt-get update to correct these problems

Workaround:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 9D1A0061
sudo apt-get update