A página de manual de add-apt-repository
diz:
REPOSITORY can be either a line that can be added directly to
sources.list(5), in the form ppa:<user>/<ppa-name> for adding Personal
Package Archives, or a distribution component to enable.
In the first form, REPOSITORY will just be appended to
/etc/apt/sources.list.
Então, precisamos usar a linha de fonte completa:
sudo add-apt-repository "deb http://repos.lpm.org.ru/kumir2/ubuntu trusty universe"