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Problem 789

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Minimal pairing modulo $p$

Given an odd prime $p$, put the numbers $1,\dots ,p-1$ into $\frac{p-1}{2}$ pairs such that each number appears exactly once. Each pair $(a,b)$ has a cost of $abmodp$. For example, if $p=5$ the pair $(3,4)$ has a cost of $12mod5=2$.

The total cost of a pairing is the sum of the costs of its pairs. We say that such pairing is optimal if its total cost is minimal for that $p$.

For example, if $p=5$, then there is a unique optimal pairing: $(1,2),(3,4)$, with total cost of $2+2=4$.

The cost product of a pairing is the product of the costs of its pairs. For example, the cost product of the optimal pairing for $p=5$ is $2\cdot 2=4$.

It turns out that all optimal pairings for $p=2000000011$ have the same cost product.

Find the value of this product.

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模$p$余数的最优配对

对于给定的奇质数$p$，将整数$1,\dots ,p-1$分成$\frac{p-1}{2}$对并保证每个整数只出现一次。每一对$(a,b)$的成本为$abmodp$。例如，若$p=5$，那么整数对$(3,4)$的成本为$12mod5=2$。

一组配对的总成本是其中每一整数对的成本之和。对于$p$，我们称总成本最小的配对为最优配对。

例如，若$p=5$，存在唯一的最优配对$(1,2),(3,4)$，其总成本为$2+2=4$。

一组配对的成本积是其中每一整数对的成本的乘积。例如，上述$p=5$的最优配对的成本积是$2\cdot 2=4$。

已知$p=2000000011$的所有最优配对都有相同的成本积。

求该成本积的值。

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