# Number of stereoisomers of pentane‐2,3,4‐triol [duplicate]

What will be the number of stereoisomers of $$\ce{CH3CH(OH)CH(OH)CH(OH)CH3}?$$

By formula $$2^n - 1 + 2^n + 1/2$$ for $$n$$ being the odd number of chiral centres, it's $$6;$$ four optically active and two optically inactive.

Optically active when RRS, SSR, RR, SS, but I am not getting how we get two optically inactive.