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;
//         for(int i = 1; i <= n; i++) {
//             sum += 1.0 / (Math.pow(-1, i - 1) * (2 * i - 1));
//         } // 测试不通过
        for(int i = 1; i <= n; i++) {
            if(i % 2 == 1) {
                sum += 1.0 / i;
            } else {
                sum -= 1.0 / i;
            }
        }
        System.out.printf("%.3f", sum);
    }
}