Let $a, b$ be two non-zero real numbers. If $p$ and $r$ are the roots of the equation $x^2-8 a x+2 a=0$ and $q$ and $s$ are the roots of the equation $x^2+12 b x+6 b=0$, such that $\frac{1}{p}, \frac{1}{q}, \frac{1}{r}, \frac{1}{s}$ are in A.P., then $a^{-1}-b^{-1}$ is equal to $\_\_\_\_$ .
