sudo vim /etc/apt/sources.list

deb http://ftp.cuhk.edu.hk/pub/Linux/ubuntu/xenial main restricted
deb http://ftp.cuhk.edu.hk/pub/Linux/ubuntu/xenial-updates main restricted
deb http://ftp.cuhk.edu.hk/pub/Linux/ubuntu/xenial universe
deb http://ftp.cuhk.edu.hk/pub/Linux/ubuntu/xenial-updates universe
deb http://ftp.cuhk.edu.hk/pub/Linux/ubuntu/xenial multiverse
deb http://ftp.cuhk.edu.hk/pub/Linux/ubuntu/xenial-updates multiverse
deb http://ftp.cuhk.edu.hk/pub/Linux/ubuntu/xenial-backports main restricted universe multiverse
deb http://ftp.cuhk.edu.hk/pub/Linux/ubuntu/xenial-security main restricted
deb http://ftp.cuhk.edu.hk/pub/Linux/ubuntu/xenial-security universe
deb http://ftp.cuhk.edu.hk/pub/Linux/ubuntu/xenial-security multiverse