#define _USE_MATH_DEFINES
#include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <complex>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <limits>
#include <climits>
#include <cfloat>
#include <functional>
using namespace std;

int main()
{
    int n;
    cin >> n;

    double ans = 2.0;
    for(int x=3; x<=n; ++x){
        int sum = 0;
        int cnt = 0;
        for(int a=1; a<x; ++a){
            for(int b=1; b<a; ++b){
                sum += a * b;
                ++ cnt;
            }
        }
        ans += 2.0 / x + ((x - 2.0) / x) * (sum / (double)cnt);
    }
    printf("%.10f\n", ans);

    return 0;
}