题解 | #牛牛学数列3#

import java.util.Scanner;
public class Main{
    public static void main(String[] args){
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        double sum = 0.0; //记录每轮的结果
        int fm = 0; //记录每轮的分母
        for(int i = 1; i <= n; i++){
            int j;
            if(i % 2 == 0 ){
                j = (-1) * (2 * i - 1);
                fm += j;
            }else{
                j = 2 * i - 1;
                fm += j;    
            }
            
            sum += 1.0 / fm;
        }
        System.out.println(String.format("%.3f",sum));
    }
}