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

#define rep(i,n) for (int i=0;i<(n);i++)
#define rep2(i,a,b) for (int i=(a);i<(b);i++)
#define rrep(i,n) for (int i=(n)-1;i>=0;i--)
#define rrep2(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define all(a) (a).begin(),(a).end()

typedef long long ll;
typedef pair<int, int> P;
typedef vector<int> vi;
typedef vector<P> vp;
typedef vector<ll> vll;

signed main() {
    int N;
    cin >> N;
    double ans = 2;
    rep2(k, 3, N + 1) {
        ans += 2.0 * 1 / k;
        double temp = 0;
        rep2(j, 1, k) {
            rep2(i, 1, j) {
                temp += i * j;
            }
        }
        temp /= 1.0 * (k - 1) * (k - 2) / 2;
        temp *= (1 - 2.0 / k);
        ans += temp;
    }
    printf("%.10f\n", ans);

}