The conductivity of a weak acid HA of concentration 0.001 mol L^$-$1 is 2.0 $\times$ 10^$-$5 S cm^$-$1. If $\Lambda _m^o$(HA) = 190 S cm² mol^$-$1, the ionization constant (K_a) of HA is equal to ______________ $\times$ 10^$-$6. (Round off to the Nearest Integer)