#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); }