题解 | #牛牛学数列2#

import java.util.*;

public class Main{
    public static void main(String[] args)throws Exception{
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
           System.out.printf("%.3f",sum(n));
    }
    
    public static double sum(int n){
        double sum = 0;
        double sum_total = 0;
        int c = -1;
       for(int i = 1; i <= n; i++){
           c *= -1;
          sum += (2 * i - 1) * c;
          sum_total += 1.0 / sum;
       }
        return sum_total;
    }
}