Very, very bad notation. Please never write that again. Add parentheses where appropriate.

Perhaps this: [(ΣY)^2 - (ΣY^2)/N]/(N-1)

Your arithemetic looks good, but your formula doesn't. Please check that and try again. It appears to me that you have switched (ΣY)^2 and (ΣY^2)

Perhaps this: [(ΣY^2) - {(ΣY)^2}/N]/(N-1)