Two discs of moment of inertia $I_1=4 \mathrm{~kg} \mathrm{~m}^2$ and $I_2=2 \mathrm{~kg} \mathrm{~m}^2$, about their central axes & normal to their planes, rotating with angular speeds $10 \mathrm{~rad} / \mathrm{s}$ & $4 \mathrm{~rad} / \mathrm{s}$ respectively are brought into contact face to face with their axes of rotation coincident. The loss in kinetic energy of the system in the process is _________ J.