|Codeforces Round #388 (Div. 2)|
You are given a permutation of integers from 1 to n. Exactly once you apply the following operation to this permutation: pick a random segment and shuffle its elements. Formally:
Inversion if a pair of elements (not necessary neighbouring) with the wrong relative order. In other words, the number of inversion is equal to the number of pairs (i, j) such that i < j and ai > aj. Find the expected number of inversions after we apply exactly one operation mentioned above.
The first line contains a single integer n (1 ≤ n ≤ 100 000) — the length of the permutation.
The second line contains n distinct integers from 1 to n — elements of the permutation.
Print one real value — the expected number of inversions. Your answer will be considered correct if its absolute or relative error does not exceed 10 - 9.
Namely: let's assume that your answer is a, and the answer of the jury is b. The checker program will consider your answer correct, if .
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