#include <bits/stdc++.h>

using namespace std;

namespace {

    typedef long double real;
    typedef long long ll;

    template<class T> ostream& operator<<(ostream& os, const vector<T>& vs) {
        if (vs.empty()) return os << "[]";
        auto i = vs.begin();
        os << "[" << *i;
        for (++i; i != vs.end(); ++i) os << " " << *i;
        return os << "]";
    }
    template<class T> istream& operator>>(istream& is, vector<T>& vs) {
        for (auto it = vs.begin(); it != vs.end(); it++) is >> *it;
        return is;
    }

    int N;
    vector<int> A;
    void input() {
        cin >> N;
        A.resize(N);
        cin >> A;
    }

    map< tuple<int,int,int>, real > cache;
    real f(int a, int b, int c) {
        if (a < 0 || b < 0 || c < 0) return 0;
        real s = a + b + c;
        if (s == 0) return 0;
        auto key = make_tuple(a, b, c);
        if (cache.count(key)) {
            return cache[key];
        }
        //cerr << a << " " << b << " " << c << " -> " <<  (N / s) + (a / s * f(a - 1, b + 1, c) + b / s * f(a, b - 1, c + 1) + c / s * f(a, b, c - 1)) << endl;
        return cache[key] = (N / s) + (a / s * f(a - 1, b + 1, c) + b / s * f(a, b - 1, c + 1) + c / s * f(a, b, c - 1));
    }
    void solve() {
        vector<int> r(4, 0);
        for (int i = 0; i < N; i++) {
            int k = max(0, 3 - A[i]);
            r[k]++;
        }
        cout << fixed << setprecision(12) << f(r[3], r[2], r[1]) << endl;
    }
}

int main() {
    input(); solve();
    return 0;
}