The tangent at any point $P$ of the hyperbola $\dfrac{x^{2}}{a^{2}} - \dfrac{y^{2}}{b^{2}} =1$ makes an intercept $p$ between the axis of the hyperbola. $p_{1} ,p_{2}$ are the lengths of perpendiculars drawn from the focii on the normal at $P$, then $p$ is