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CSES Problem Set

Inversion Probability


Task | Statistics


CSES - Inversion ProbabilityCSES - Inversion Probability

Time limit:1.00 s Memory limit:512 MB

An array has $n$ integers $x_1,x_2,\dots,x_n$, and each of them has been randomly chosen between $1$ and $r_i$. An inversion is a pair $(a,b)$ where $a<b$ and $x_a>x_b$.

What is the expected number of inversions in the array?

Input

The first input line contains an integer $n$: the size of the array.

The second line contains $n$ integers $r_1,r_2,\dots,r_n$: the range of possible values for each array position.

Output

Print the expected number of inversions rounded to six decimal places.

Constraints
Example

Input:
3
5 2 7


Output:
1.057143