An aqueous solution contains $0.10 \, M \, H_2S$ and $0.20 \, M \, HCl$ . If the equilibrium constants for the formation of $HS^-$ from $H_2S$ is $1.0 \times 10^{-7}$ and that of $S^{2- }$ from $HS^-$ ions is $ 1.2 \times 10^{-13}$ then the concentration of $S^{2-}$ ions in aqueous solution is -