The compound shown is hex-3-ene (no geometry specified). How many open-chain alkenes of molecular formula $\mathrm{C}{}_6\mathrm{H}{}_{12}$ exist in total, counting constitutional isomers, geometrical isomers and enantiomers all separately? Give the answer as an integer.

Constitutional isomers: hexane skeleton gives hex-1-ene, hex-2-ene, hex-3-ene (3); 2-methylpentane skeleton gives 2-methylpent-1-ene, 2-methylpent-2-ene, 4-methylpent-1-ene, 4-methylpent-2-ene (4); 3-methylpentane skeleton gives 3-methylpent-1-ene, 3-methylpent-2-ene, 2-ethylbut-1-ene (3); 2,3-dimethylbutane skeleton gives 2,3-dimethylbut-1-ene and 2,3-dimethylbut-2-ene (2); 2,2-dimethylbutane skeleton gives 3,3-dimethylbut-1-ene (1). Total 13. Geometrical isomerism doubles hex-2-ene, hex-3-ene, 4-methylpent-2-ene and 3-methylpent-2-ene (+4). 3-Methylpent-1-ene has a stereocentre at C3 (+1). Total = 13 + 4 + 1 = 18.