The conductivity of a weak acid HA of concentration 0.001 mol L$^{-1}$ is 2.0 $\times$ 10$^{-5}$ S cm$^{-1}$. If $\Lambda^\circ_m$(HA) = 190 S cm$^2$ mol$^{-1}$, the ionization constant (K$_a$) of HA is equal to ___ $\times$ 10$^{-6}$.