Xᵃ⁺ and Yᵇ⁺ 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 Xᵃ⁺ is λ. The wavelength of light absorbed during the transition between the states with principal quantum numbers n = 2 and n = 4 of Yᵇ⁺ is 9λ. The lowest possible value of (a+b) is ____ .