apt - W: GPG error: The following signatures couldn't be verified because the public key is not available

On my Ubuntu 14.04, I followed http://mirror.its.dal.ca/cran/bin/linux/ubuntu/README to add the following to /etc/apt/sources.list

deb http://stat.ethz.ch/CRAN/bin/linux/ubuntu trusty/

When I run sudo apt-get update, I get this error

W: GPG error: http://stat.ethz.ch trusty/ Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 51716619E084DAB9

The whole output is at the end of this post.
I wonder what the problem means and how to solve it? Thanks!

$ sudo apt-get update
Ign http://us.archive.ubuntu.com trusty InRelease
Ign http://us.archive.ubuntu.com trusty-updates InRelease
Ign http://dl.google.com stable InRelease
Ign http://us.archive.ubuntu.com trusty-backports InRelease
Ign http://security.ubuntu.com trusty-security InRelease
Hit http://us.archive.ubuntu.com trusty Release.gpg
Hit http://security.ubuntu.com trusty-security Release.gpg
Hit http://us.archive.ubuntu.com trusty-updates Release.gpg
Ign http://dl.google.com stable InRelease
Hit http://us.archive.ubuntu.com trusty-backports Release.gpg
Hit http://security.ubuntu.com trusty-security Release
Hit http://dl.google.com stable Release.gpg
Hit http://us.archive.ubuntu.com trusty Release
Hit http://us.archive.ubuntu.com trusty-updates Release
Hit http://security.ubuntu.com trusty-security/main Sources
Ign http://extras.ubuntu.com trusty InRelease
Hit http://dl.google.com stable Release.gpg
Hit http://us.archive.ubuntu.com trusty-backports Release
Ign http://archive.canonical.com trusty InRelease
Hit http://security.ubuntu.com trusty-security/restricted Sources
Hit http://us.archive.ubuntu.com trusty/main Sources
Hit http://dl.google.com stable Release
Hit http://us.archive.ubuntu.com trusty/restricted Sources
Hit http://security.ubuntu.com trusty-security/universe Sources
Hit http://us.archive.ubuntu.com trusty/universe Sources
Hit http://security.ubuntu.com trusty-security/multiverse Sources
Hit http://security.ubuntu.com trusty-security/main amd64 Packages
Hit http://dl.google.com stable Release
Hit http://us.archive.ubuntu.com trusty/multiverse Sources
Hit http://extras.ubuntu.com trusty Release.gpg
Hit http://security.ubuntu.com trusty-security/restricted amd64 Packages
Hit http://us.archive.ubuntu.com trusty/main amd64 Packages
Ign http://archive.canonical.com trusty InRelease
Hit http://security.ubuntu.com trusty-security/universe amd64 Packages
Hit http://us.archive.ubuntu.com trusty/restricted amd64 Packages
Hit http://dl.google.com stable/main amd64 Packages
Hit http://security.ubuntu.com trusty-security/multiverse amd64 Packages
Hit http://us.archive.ubuntu.com trusty/universe amd64 Packages
Ign http://stat.ethz.ch trusty/ InRelease
Hit http://security.ubuntu.com trusty-security/main i386 Packages
Hit http://us.archive.ubuntu.com trusty/multiverse amd64 Packages
Hit http://dl.google.com stable/main i386 Packages
Hit http://security.ubuntu.com trusty-security/restricted i386 Packages
Hit http://us.archive.ubuntu.com trusty/main i386 Packages
Hit http://us.archive.ubuntu.com trusty/restricted i386 Packages
Hit http://security.ubuntu.com trusty-security/universe i386 Packages
Hit http://extras.ubuntu.com trusty Release
Hit http://archive.canonical.com trusty Release.gpg
Hit http://security.ubuntu.com trusty-security/multiverse i386 Packages
Hit http://security.ubuntu.com trusty-security/main Translation-en
Hit http://security.ubuntu.com trusty-security/multiverse Translation-en
Hit http://us.archive.ubuntu.com trusty/universe i386 Packages
Hit http://security.ubuntu.com trusty-security/restricted Translation-en
Get:1 http://stat.ethz.ch trusty/ Release.gpg [490 B]
Hit http://dl.google.com stable/main amd64 Packages
Hit http://us.archive.ubuntu.com trusty/multiverse i386 Packages
Hit http://archive.canonical.com trusty Release.gpg
Hit http://extras.ubuntu.com trusty/main Sources
Hit http://dl.google.com stable/main i386 Packages
Hit http://us.archive.ubuntu.com trusty/main Translation-en
Hit http://security.ubuntu.com trusty-security/universe Translation-en
Hit http://us.archive.ubuntu.com trusty/multiverse Translation-en
Hit http://stat.ethz.ch trusty/ Release
Ign http://stat.ethz.ch trusty/ Release
Hit http://extras.ubuntu.com trusty/main amd64 Packages
Hit http://us.archive.ubuntu.com trusty/restricted Translation-en
Hit http://archive.canonical.com trusty Release
Hit http://us.archive.ubuntu.com trusty/universe Translation-en
Hit http://us.archive.ubuntu.com trusty-updates/main Sources
Hit http://extras.ubuntu.com trusty/main i386 Packages
Hit http://us.archive.ubuntu.com trusty-updates/restricted Sources
Hit http://archive.canonical.com trusty Release
Hit http://us.archive.ubuntu.com trusty-updates/universe Sources
Ign http://stat.ethz.ch trusty/ Packages/DiffIndex
Hit http://us.archive.ubuntu.com trusty-updates/multiverse Sources
Hit http://us.archive.ubuntu.com trusty-updates/main amd64 Packages
Hit http://us.archive.ubuntu.com trusty-updates/restricted amd64 Packages
Hit http://us.archive.ubuntu.com trusty-updates/universe amd64 Packages
Hit http://archive.canonical.com trusty/partner Sources
Hit http://us.archive.ubuntu.com trusty-updates/multiverse amd64 Packages
Hit http://us.archive.ubuntu.com trusty-updates/main i386 Packages
Ign https://private-ppa.launchpad.net precise InRelease
Hit http://archive.canonical.com trusty/partner amd64 Packages
Hit http://us.archive.ubuntu.com trusty-updates/restricted i386 Packages
Hit http://us.archive.ubuntu.com trusty-updates/universe i386 Packages
Hit http://us.archive.ubuntu.com trusty-updates/multiverse i386 Packages
Hit http://us.archive.ubuntu.com trusty-updates/main Translation-en
Hit https://private-ppa.launchpad.net precise Release.gpg
Hit http://us.archive.ubuntu.com trusty-updates/multiverse Translation-en
Hit http://us.archive.ubuntu.com trusty-updates/restricted Translation-en
Hit http://archive.canonical.com trusty/partner i386 Packages
Hit http://us.archive.ubuntu.com trusty-updates/universe Translation-en
Hit http://us.archive.ubuntu.com trusty-backports/main Sources
Hit http://us.archive.ubuntu.com trusty-backports/restricted Sources
Hit http://us.archive.ubuntu.com trusty-backports/universe Sources
Hit http://us.archive.ubuntu.com trusty-backports/multiverse Sources
Hit http://stat.ethz.ch trusty/ Packages
Hit https://private-ppa.launchpad.net precise Release
Hit http://us.archive.ubuntu.com trusty-backports/main amd64 Packages
Hit http://us.archive.ubuntu.com trusty-backports/restricted amd64 Packages
Hit http://us.archive.ubuntu.com trusty-backports/universe amd64 Packages
Hit http://us.archive.ubuntu.com trusty-backports/multiverse amd64 Packages
Hit http://us.archive.ubuntu.com trusty-backports/main i386 Packages
Hit http://us.archive.ubuntu.com trusty-backports/restricted i386 Packages
Ign http://dl.google.com stable/main Translation-en_US
Hit http://archive.canonical.com trusty/partner Sources
Hit https://private-ppa.launchpad.net precise/main amd64 Packages
Ign http://dl.google.com stable/main Translation-en
Hit http://us.archive.ubuntu.com trusty-backports/universe i386 Packages
Ign http://dl.google.com stable/main Translation-en_US
Hit http://us.archive.ubuntu.com trusty-backports/multiverse i386 Packages
Hit http://us.archive.ubuntu.com trusty-backports/main Translation-en
Hit http://archive.canonical.com trusty/partner amd64 Packages
Hit https://private-ppa.launchpad.net precise/main i386 Packages
Hit http://us.archive.ubuntu.com trusty-backports/multiverse Translation-en
Ign http://dl.google.com stable/main Translation-en
Hit http://us.archive.ubuntu.com trusty-backports/restricted Translation-en
Hit http://us.archive.ubuntu.com trusty-backports/universe Translation-en
Hit http://archive.canonical.com trusty/partner i386 Packages
Ign http://extras.ubuntu.com trusty/main Translation-en_US
Ign http://us.archive.ubuntu.com trusty/main Translation-en_US
Ign http://us.archive.ubuntu.com trusty/multiverse Translation-en_US
Ign http://us.archive.ubuntu.com trusty/restricted Translation-en_US
Ign http://extras.ubuntu.com trusty/main Translation-en
Ign http://us.archive.ubuntu.com trusty/universe Translation-en_US
Ign http://archive.canonical.com trusty/partner Translation-en
Ign http://stat.ethz.ch trusty/ Translation-en_US
Ign http://archive.canonical.com trusty/partner Translation-en
Ign http://stat.ethz.ch trusty/ Translation-en
Ign https://private-ppa.launchpad.net precise/main Translation-en_US
Ign https://private-ppa.launchpad.net precise/main Translation-en
Fetched 490 B in 6s (78 B/s)
Reading package lists... Done
W: GPG error: http://stat.ethz.ch trusty/ Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 51716619E084DAB9

See the README file:

SECURE APT

The Ubuntu archives on CRAN are signed with the key of "Michael Rutter
<...@....com>" with key ID E084DAB9. To add the key to your system
with one command use (thanks to Brett Presnell for the tip):

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys E084DAB9

An alternate method can be used by retrieving the key with

gpg --keyserver keyserver.ubuntu.com --recv-key E084DAB9

and then feed it to apt-key with

gpg -a --export E084DAB9 | sudo apt-key add -