The compound shown is 2,4-dibromo-3-chloropentane drawn without stereochemistry. How many of its stereoisomers are meso (optically inactive) compounds? Give an integer.

C2 and C4 are stereocentres with identical substituent sets. When they have opposite descriptors, $(2R,4S)$, the molecule has a mirror plane through C3, and C3 then becomes pseudoasymmetric: its $\mathrm{Cl}$ can point to either side relative to the two enantiomeric arms, giving two distinct meso compounds. When C2 and C4 are both $R$ or both $S$, C3 carries two identical groups and is not a stereocentre; this gives one pair of enantiomers. So there are 4 stereoisomers in total, 2 of them meso.