#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>
#include <numeric>
#include <functional>
#include <cmath>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <sstream>
#include <string>
#define repd(i,a,b) for (int i=(int)(a);i<(int)(b);i++)
#define rep(i,n) repd(i,0,n)
#define all(x) (x).begin(),(x).end()
#define mod 1000000007
#define inf 2000000007
#define mp make_pair
#define pb push_back
typedef long long ll;
using namespace std;
template <typename T>
inline void output(T a, int p) {
    if(p) cout << fixed << setprecision(p)  << a << "\n";
    else cout << a << "\n";
}
// end of template

int main() {
    cin.tie(0);
    ios::sync_with_stdio(0);
    // source code
    vector<ll> fact(21, 1);
    repd(i, 2, 21){
        fact[i] = fact[i - 1] * i;
    }
    vector<double> ret(21, 0);
    ret[2] = 2;
    repd(i, 3, 21){
        ll tmp = 0;
        repd(j, 1, i){
            repd(k, j + 1, i){
                tmp += j * k;
            }
        }
        
//        tmp *= 2 * fact[i - 2];
//        tmp += 2 * fact[i - 1];
//        ret[i] = (double)tmp / (double)fact[i];
        ret[i] = (double)tmp * 2.0 / (double)((i - 1) * i);
        ret[i] += 2.0 / (double)i;
        ret[i] += ret[i - 1];
    }
    int N;
    cin >> N;
    output(ret[N], 10);
    
    return 0;
}