//
//
//  Codeforces: ganariya
//  AtCoder: ganariya2525
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//include
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <unordered_map>
#include <climits>
#include <set>
#include <unordered_set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <queue>
#include <random>
#include <complex>
#include <regex>
#include <locale>
#include <random>
#include <type_traits>

using namespace std;

#define SHOW_VECTOR(v) {std::cerr << #v << "\t:";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOW_MAP(v){std::cerr << #v << endl; for(const auto& xxx: v){std::cerr << xxx.first << " " << xxx.second << "\n";}}

using LL = long long;

//~~~~~~~~~~~~~~~~~~~~~_(＾～＾ ｣ ∠)_~~~~~~~~~~~~~~~~~~~~~

constexpr int MAX_N = 310;

double f(int one, int two, int three, vector<vector<vector<double>>> &dp, int N) {
    if (dp[one][two][three] >= 0.0)return dp[one][two][three];
    if (one == 0 && two == 0 && three == 0) return dp[one][two][three] = 0.0;

    double res = 0.0;
    if (one > 0) res += f(one - 1, two, three, dp, N) * one;
    if (two > 0) res += f(one + 1, two - 1, three, dp, N) * two;
    if (three > 0) res += f(one, two + 1, three - 1, dp, N) * three;

    res += N;

    res *= 1.0 / (double) (one + two + three);

    return dp[one][two][three] = res;
}


int main() {

    int N;
    cin >> N;

    vector<vector<vector<double>>>
            dp(MAX_N, vector<vector<double>>(MAX_N, vector<double>(vector<double>(MAX_N, -1))));

    int one = 0;
    int two = 0;
    int three = 0;

    for (int i = 0; i < N; i++) {
        int x;
        cin >> x;
        if (x == 1)one++;
        if (x == 2) two++;
        if (x == 3)three++;
    }

    cout << fixed << setprecision(20) << f(one, two, three, dp, N);

    return 0;
}
















































Measured memory_bytes: 237568