题解 | #牛牛学数列3#

import java.util.Scanner;

/**
 * @author zmstart
 * @create 2022-03-20 17:09
 */
public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        double sum = 0;

        for (int i = 1; i <= n; i++) {
            double a = 0;
            for (int j = 1; j <= i; j++) {
                if (j % 2 == 0) {
                    a -= (2 * j - 1);
                } else {
                    a += (2 * j - 1);
                }
            }
            sum += (1 / a);
        }
       
        System.out.printf("%.3f",sum);
    }
}