\[1)\ P_{2n} = (2n)! - всего\ рядов.\]
\[2)\ Разобьем\ ряд\ на\ \text{n\ }пар:\]
\[|1,\ 2|,\ |3,\ 4|,\ |5,\ 6|,\ \ldots,\ |2n - 1,\ 2n|.\]
\[3)\ A = \frac{P_{2n}}{2 \bullet 2 \bullet 2 \bullet \ldots \bullet 2} = \frac{(2n)!}{2^{n}}.\]
\[4)\ C = \frac{A}{P_{n}} = \frac{(2n)!}{2^{n} \bullet n!}.\]
\[Ответ:\ \ \frac{(2n)!}{2^{n} \bullet n!}.\]