For the titration of weak dibasic acid $H_{2}A$ (conical flask) against $NaOH,$ graph is given  below. To the one litre of $0.1M{H}_{2}A,$ when 2g, 4g and 6 g of NaOH is added, pH of the mixtures are respectively X, Y and Z. (X+Y+Z) value is ( ${\left[H_{2}A\right]}_0$ is the initial concentration of $H_{2}A. \left[H_{2}A\right]$ is concentration of $H_{2}A$ at that instant,  $\left[H{A}^{-}\right]$ and $\left[A^{2-}\right]$ are the concentrations of $H{A}^{-}$ and $A^{2-}$ at that instant.)