Structure Of Atom

$\mathbf{X}^{\boldsymbol{a}+}$ and $\mathbf{Y}^{\boldsymbol{b}+}$ are hydrogen-like species. The wavelength of light absorbed during the transition between the states with principal quantum numbers $n=1$ and $n=2$ of $\mathbf{X}^{\boldsymbol{a}+}$ is $\lambda$. The wavelength of light absorbed during the transition between the states with principal quantum numbers $n=2$ and $n=4$ of $\mathbf{Y}^{\boldsymbol{b}+}$ is $9 \lambda$. The lowest possible value of $(\boldsymbol{a}+\boldsymbol{b})$ is $\_\_\_\_$ .