403021: GYM100971 B Derangement

A permutation of n numbers is a sequence of integers from 1 to n where each number is occurred exactly once. If a permutation p₁, p₂, ..., p_n has an index i such that p_i = i, this index is called a fixed point.

A derangement is a permutation without any fixed points.

Let's denote the operation swap(a, b) as swapping elements on positions a and b.

For the given permutation find the minimal number of swap operations needed to turn it into derangement.

The first line contains an integer n (2 ≤ n ≤ 200000) — the number of elements in a permutation.

The second line contains the elements of the permutation — n distinct integers from 1 to n.

In the first line output a single integer k — the minimal number of swap operations needed to transform the permutation into derangement.

In each of the next k lines output two integers a_i and b_i (1 ≤ a_i, b_i ≤ n) — the arguments of swap operations.

If there are multiple possible solutions, output any of them.

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