//
//
//  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 N, int one, int two, int three, vector<vector<vector<double>>> &dp) {
    if (one == 0 && two == 0 && three == 0) {
        return dp[one][two][three] = 0;
    }
    if (dp[one][two][three] >= -0.1)return dp[one][two][three];
    dp[one][two][three] = 0;
    double s = one + two + three;
    if (one >= 1) dp[one][two][three] += f(N, one - 1, two, three, dp) * ((double) one / s);
    if (two >= 1) dp[one][two][three] += f(N, one + 1, two - 1, three, dp) * ((double) two / s);
    if (three >= 1) dp[one][two][three] += f(N, one, two + 1, three - 1, dp) * ((double) three / s);
    dp[one][two][three] += N / s;
    return dp[one][two][three];
}

int main() {

    int N;
    cin >> N;

    vector<int> a(N);
    for (int i = 0; i < N; i++) cin >> a[i];

    int one = count(a.begin(), a.end(), 1);
    int two = count(a.begin(), a.end(), 2);
    int three = count(a.begin(), a.end(), 3);

    vector<vector<vector<double>>> dp(MAX_N, vector<vector<double>>(MAX_N, vector<double>(MAX_N, -1)));
    double ans = f(N, one, two, three, dp);

    cout << fixed << setprecision(20) << ans;

}






































Measured memory_bytes: 237568