#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef vector< int > vi;
typedef vector< vi > vvi;
typedef vector< ll > vl;
typedef vector< vl > vvl;
typedef pair< int, int > pii;
typedef vector< pii > vp;
typedef vector< double > vd;
typedef vector< vd > vvd;
typedef vector< string > vs;

template< class T1, class T2 >
int upmin( T1 &x, T2 v ){
  if( x > v ){
    x = v;
    return 1;
  }
  return 0;
}

template< class T1, class T2 >
int upmax( T1 &x, T2 v ){
  if( x < v ){
    x = v;
    return 1;
  }
  return 0;
}

const int INF = 0x3f3f3f3f;

int N;

void init(){
  cin >> N;
}

void preprocess(){

}

void solve(){
  double ans = 1.0;
  for( int i = 2; i <= N; ++i ){
    ans += 2.0 / i;
    for( int j = 1; j < i; ++j )
      for( int k = j + 1; k < i; ++k )
        ans += 1.0 * j * k / ( ( i - 1 ) * ( i - 2 ) / 2 ) * ( i - 2 ) / i;
  }
  cout << fixed << setprecision( 9 ) << ans << endl;
}

signed main(){
  ios::sync_with_stdio( 0 );
  init();
  preprocess();
  solve();
  return 0;
}