#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
//#include<cctype>
#include<climits>
#include<iostream>
#include<string>
#include<vector>
#include<map>
//#include<list>
#include<queue>
#include<deque>
#include<algorithm>
//#include<numeric>
#include<utility>
#include<complex>
//#include<memory>
#include<functional>
#include<cassert>
#include<set>
#include<stack>

const int dx[] = {1, 0, -1, 0};
const int dy[] = {0, 1, 0, -1};
using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef pair<int, int> pii;

int main() {
    int N;
    cin >> N;
    double ans = 0;
    for (int i = 1; i <= N; i++) {
        if (i <= 2) ans += 1;
        else {
            double plus = 2./i;
            int sum = 0;
            for (int j = 1; j <= i-1; j++) for (int k = j+1; k <= i-1; k++) {
                sum += j*k;
            }
            plus += 2.*sum/i/(i-1);
            ans += plus;
        }
    }
    printf("%.10lf\n", ans);
    return 0;
}