A circular disc of mass M and radius R is rotating about its axis with angular speed $\omega _ { 1 }$. If another stationary disc having radius $\frac { \mathrm { R } } { 2 }$ and same mass M is dropped co-axially on to the rotating disc. Gradually both discs attain constant angular speed $\omega _ { 2 }$. The energy lost in the process is $p \%$ of the initial energy. Value of $p$ is $\_\_\_\_$