A protein, shown schematically in Figure 6.30, may contain several cysteines (represented as light gray balls), which may pair together to form disulfide bonds. For six cysteines, three disulfide bonds can form; the figure shows the pairing arrangement 1-6, 2-5, 3-4. How many different disulfide pairing arrangements are possible? Derive the general formula for the number of different pairing arrangements when there are n cysteines (and n is even).

1 2 3 6 5 Figure 6.30 Physical Biology of the Cell, 2ed. ( Garland Science 2013)