import java.util.Scanner;


public class Main_yukicoder352 {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();

//        long[] fact = new long[n + 1];
//        fact[0] = 1;
//        for (int i = 1; i <= n; i++) {
//        	fact[i] = fact[i - 1] * i;
//        }

        double ret = 2;
        for (int i = 3; i <= n; i++) {
        	int m = i - 1;
        	double tmp = (double)2 / (m + 1);
        	for (int j = 1; j <= m; j++) {
        		for (int k = j + 1; k <= m; k++) {
        			if (j == k) {
        				continue;
        			}

        			tmp += (double)2 * j * k / m / (m + 1);
        		}
        	}

        	ret += tmp;
        }

        System.out.printf("%.7f\n", ret);

        sc.close();
    }
}