Rate the following ions in increasing ionization energy

Question

Sr²⁺, Rb¹⁺, S^2-, P^3-

I believe it is P^3- < S^2- < Rb¹⁺ < Sr²⁺ because it should be easier to take an electron away from the negatively charged ions, but I'm not sure about the Rb and Sr because they both have their electrons taken out of their outer shell.