A wooden plank of length $1 \mathrm{~m}$ and uniform cross-section is hinged at one end to the bottom of a tank as shown in Fig. $8.48$. The tank is filled with water up to a height of $0.5 \mathrm{~m}$. The specific gravity of the plank is $0.5$. The angle $\theta$ in radian that the plank makes with the vertical in the equilibrium position is $\frac{\pi}{n}$, then the value of $n$ is.