GPG error

problem:
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W: GPG error: http://dl.google.com stable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A040830F7FAC5991
W: You may want to run apt-get update to correct these problems



solution:
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flufyz$ gpg --keyserver dl.google.com --recv A040830F7FAC5991
flufyz$ gpg --export --armor A040830F7FAC5991 | sudo apt-key add -