A protein shown schematically in the figure, may contain several cysteins (presented as light gray balls), which may pair together to form disulfide bonds. For six cysteins, 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 a general formula for the number of different pairing arrangements when there are n cysteins (and n is an even number).