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 $\frac{R}{2}$ 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 $\mathrm{p} \%$ of the initial energy. Value of p is $\_\_\_\_$ .