#include<iostream>
#include<cstdio>
using namespace std;
const int BUF = 25;


int n;

void read() {
    cin >> n;
}


double bino(int n, int k) {
    return k == 0 ? 1 : bino(n - 1, k - 1) * n / k;
}

void work() {
    
    double dp[BUF];
    dp[1] = 1;
    
    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) {
                if (j == k) continue;
                sum += j * k;
            }
        dp[i] = dp[i - 1] + 2.0 / i + sum / (i == 2 ? 1 : bino(i - 1, 2)) * (i - 2) / i;
    }
    
    printf("%.10lf\n", dp[n]);
}


int main() {
    read();
    work();
    return 0;
}