/* -*- coding: utf-8 -*-
 *
 * 352.cc: No.352 カード並べ - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 20;

/* typedef */

/* global variables */

/* subroutines */

/* main */

// when putting card i,
// e(i) = 1 * 2/n + sigma_{i,j} (i*j) * 2*(n-2)!/(n-1)! * 1/n
int main() {
  int n;
  cin >> n;

  double e = 1.0;
  for (int i = 2; i <= n; i++) {
    int sum = 0;
    for (int j = 1; j < i; j++)
      for (int k = j + 1; k < i; k++) sum += j * k;

    double ei = (2.0 + sum * 2.0 / (i - 1)) / i;
    //printf("%d: ei=%.lf\n", i, ei);
    e += ei;
  }

  printf("%.10lf\n", e);
  return 0;
}