#include <iostream>
#include <cstdio>
#define repeat_from(i,m,n) for (int i = (m); (i) < (n); ++(i))
typedef long long ll;
using namespace std;
int main() {
    int n; cin >> n;
    double ans = 0;
    ans += 1; // 1
    ans += 1; // 2
    repeat_from (i,3,n+1) {
        ll p = 0;
        int q = 0;
        repeat_from (a,1,i) repeat_from (b,a+1,i) {
            p += a * b;
            q += 1;
        }
        ans += (2 + (p /(double) q) * (i-2)) / i;
    }
    printf("%.10lf\n", ans);
    return 0;
}