#include <iostream>
#include <vector>
#include <map>
#include <algorithm>
#include <iomanip>
using namespace std;

#define REP(i,s,e) for (i = s; i <= e; i++)
#define rep(i,n) REP (i,0,(int)(n)-1)
#define RREP(i,s,e) for (i = s; i >= e; i--)
#define rrep(i,n) RREP (i,(int)(n)-1,0)
#define INF (int)1e8
#define MOD (int)(1e9+7)

typedef long long ll;

int main(void) {
    int i, j, k, n;

    double ans = 1;
    cin >> n;
    REP (i,2,n) {
        ans += 2. / i;
        double plus = 0;
        int cnt = 0;
        REP (j,1,i-2) {
            REP (k,j+1,i-1) {
                plus += j * k;
                cnt++;
            }
        }
        if (i > 2) plus = plus * (i - 2) / i / cnt;
        ans += plus;
    }
    cout << fixed << setprecision(7) << ans << endl;
    return 0;
}