Let $X_1,...,X_n\sim$Poiss($\lambda$) i.i.d. and $S^2_n=(n-1)^{-1}\sum\limits_{i=1}^n(X_i-\bar X_i)^2$ where $\bar X_n={1\over n}\sum\limits_{i=1}^nX_i$
How would you calculate $\Bbb E[S_n^2]$ the expected value of $S_n^2$?
I would show you what I tried but I have no idea how to do this