#include <bits/stdc++.h>

using namespace std;

#define FOR(i,n) for(int i = 0 ;i < (n); i++) 
#define sz(c) ((int)c.size())
#define ten(n) ((int)1e##n)

typedef long long ll;
typedef pair<ll, ll> Pll;
typedef pair<int, int> Pii;

int main() {
	int n; cin >> n;
	double ans = 1.0;
	for (int c = 2; c <= n; c++) {
		double p = 2 / double(c * (c - 1));
		double sum = 0;
		for (int x = 1; x < c; x++) {
			for (int y = 1; y < x; y++) {
				sum += x * y;
			}
		}
		ans += sum * p + 2.0 / c;
	}

	printf("%.15lf\n",ans);

	return 0;
}