#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <queue>
#include <string>
#include <map>
#include <set>
#include <stack>
#include <tuple>
#include <deque>
#include <array>
#include <numeric>
#include <bitset>
#include <iomanip>
#include <cassert>
#include <chrono>
#include <random>
#include <limits>
#include <iterator>
#include <functional>
#include <sstream>
#include <fstream>
#include <complex>
#include <cstring>
#include <unordered_map>
#include <unordered_set>
using namespace std;

using ll = long long;
constexpr int INF = 1001001001;
// constexpr int mod = 1000000007;
constexpr int mod = 998244353;

template<class T>
inline bool chmax(T& x, T y){
    if(x < y){
        x = y;
        return true;
    }
    return false;
}
template<class T>
inline bool chmin(T& x, T y){
    if(x > y){
        x = y;
        return true;
    }
    return false;
}

double dp[101][101][101];

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int N;
    cin >> N;
    vector<int> cnt(3);
    for(int i = 0; i < N; ++i){
        int A;
        cin >> A;
        if(A < 3)   cnt[A] += 1;
    }
    
    for(int i = 0; i <= N; ++i){
        for(int j = 0; j <= N; ++j){
            for(int k = 0; k <= N; ++k){
                dp[i][j][k] = -1;
            }
        }
    }

    auto func = [&](auto&& self, int i, int j, int k) -> double {
        if(dp[i][j][k] != -1)   return dp[i][j][k];
        if(i == 0 && j == 0 && k == 0)  return dp[i][j][k] = 0;
        double res = 0;
        if(i > 0)   res += self(self, i - 1, j + 1, k) * i;
        if(j > 0)   res += self(self, i, j - 1, k + 1) * j;
        if(k > 0)   res += self(self, i, j, k - 1) * k;
        res += N;
        res /= i + j + k;
        return dp[i][j][k] = res;
    };

    cout << fixed << setprecision(15);
    cout << func(func, cnt[0], cnt[1], cnt[2]) << endl;

    return 0;
}