text: For certain classes of molecules, the general formula can be simplified or rewritten more clearly. For example:
  where
  :a = number of carbon atoms in the compound
  :b = number of hydrogen atoms in the compound
  :c = number of nitrogen atoms in the compound
  :f = number of halogen atoms in the compound
  or
  where C = number of carbons, H = number of hydrogens, X = number of halogens and N = number of nitrogens, gives an equivalent result.
  In either case, oxygen and other divalent atoms do not contribute to the degree of unsaturation, as 2 − 2 = 0.