Java :

import java.util.Scanner;

public class Main{
	public static void main(String[] args) {
	  Scanner input = new Scanner(System.in);

int max = input.nextInt();
double item = 0;
double sum = 0;
int flag = -1;
for (int n = 0; n <= max; n++) {
	flag *= -1;
	item = flag * 1.0 / (2 * n + 1);
	sum += item;
}
System.out.println( sum * 4);

}

}