Prove that recurrence in 2D Random Walk only if $\space p=q$ and $r=s$

Question

I am looking on proving that 2D random walks are recurrent if and only if $p=q$ and $r=s$.

I can't manage the proof of the only if part. Have tried using convergence tests and creating bounds but no good. Please help. You may also send link to an article containing an elementary approach to this.