题解 | #牛牛学数列3#

import java.util.Scanner;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
       Scanner sc = new Scanner(System.in);
       int n = sc.nextInt();
       double sum = 0;
       for (double i = 1;i<=n;i++){
        if((int)i%2==0){
            sum -=1/i;
        }else{
            sum +=1/i;
        }
       }
       System.out.printf("%.3f",sum);

    }
}