# Sum of inverse squares

Medium

Execution time limit is 1 second

Runtime memory usage limit is 128 megabytes

Given value of n, find the sum

## Input

One positive integer n (n ≤ `10^9`

).

## Output

Print the value of the sum with 6 decimal digits.

## Examples

Input #1

Answer #1

Input #11

Answer #11

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Acceptance rate 17%