Sum of the First N Terms of an Alternating Fraction Sequence

Problem Description

Compute the sum of the first $N$ terms of the alternating sequence $1 - \frac{2}{3} + \frac{3}{5} - \frac{4}{7} + \frac{5}{9} - \frac{6}{11} + \dots$.

Input Format

A single line containing a positive integer $N$.

Output Format

Output the value of the partial sum on one line, kept to three decimal places.

Sample Input

5

Sample Output

0.917

Constraints

$1 \le N \le 1000$