The compound shown is 1,2-dimethylcyclobutane with no configuration specified. Treating the ring as planar and counting configurational stereoisomers only, how many stereoisomers does it have? Give the answer as an integer.

The two adjacent stereocentres carry identical substituents. The cis isomer has a mirror plane that bisects the C1-C2 and C3-C4 bonds, so it is a meso compound. The trans isomer lacks this plane and is a pair of enantiomers, (1R,2R) and (1S,2S). Total stereoisomers = 1 + 2 = 3.