#define _USE_MATH_DEFINES
#include <algorithm>
#include <cstdio>
#include <functional>
#include <iostream>
#include <cfloat>
#include <climits>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
using namespace std;

typedef long long ll;

int MOD = 1000000007;

int main() {
	int N; cin >> N;
	double ans = 2;
	for (int x = 3; x <= N; x++) {
		ans += 2.0 / x;
		int cnt = 0, sum = 0;
		for (int y = 1; y < x; y++)
			for (int z = y + 1; z < x; z++) {
				cnt++;
				sum += y * z;
			}
		ans += (1 - 2.0 / x) * sum / cnt;
	}
	printf("%.15f\n", ans);
}