The compound shown is 1,2-dimethylcyclopropane (no configuration specified). Count all isomers of its molecular formula $\mathrm{C}{}_5\mathrm{H}{}_{10}$: all open-chain and cyclic constitutional isomers together with all geometrical and optical isomers (enantiomers counted separately). Give the answer as an integer.

Open-chain alkenes: 5 constitutional isomers, of which pent-2-ene gives cis and trans forms, so 6. Cyclic isomers: cyclopentane, methylcyclobutane, 1,1-dimethylcyclopropane, ethylcyclopropane and 1,2-dimethylcyclopropane (5 constitutional isomers). Of these, only 1,2-dimethylcyclopropane has stereoisomers: cis (meso) and a pair of trans enantiomers, 3 in all. Cyclic total = 4 + 3 = 7. Grand total = 6 + 7 = 13.