The solubility of a salt of weak acid ( $\mathbf{A B}$ ) at pH 3 is $\mathbf{Y} \times 10^{-3} \mathrm{~mol} \mathrm{~L}^{-1}$. The value of $\mathbf{Y}$ is $\_\_\_\_$。 (Given that the value of solubility product of $\mathbf{A B}\left(\mathrm{K}_{\mathrm{sp}}\right)=2 \times 10^{-10}$ and the value of ionization constant of HB $\left.\left(K_a\right)=1 \times 10^{-8}\right)$