#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>
#include <numeric>
#include <functional>
#include <cmath>
#include <queue>
#include <stack>
#include <set>
#include <map>

#define repd(i,a,b) for (int i=(a);i<(b);i++)
#define rep(i,n) repd(i,0,n)
#define var auto
#define mod 1000000007
typedef long long ll;
using namespace std;
typedef vector<double> vd;
typedef vector<vector<double>> vvd;
typedef vector<vector<vector<double>>> vvvd;


int inputValue(){
    int a;
    cin >> a;
    return a;
}

template <typename T>
void output(T a, int precision) {
    if(precision > 0){
        cout << fixed << setprecision(precision)  << a << "\n";
    }
    else{
        cout << a << "\n";
    }
}

double dp[101][101][101]; // 0
int N;

double dfs(int i, int j, int k){
    if (dp[i][j][k] != 0) {
        return dp[i][j][k];
    }
    int sum = i + j + k;
    if (sum == 0) {
        return dp[i][j][k] = 0;
    }
    
    double ret = 0;
    
    if (i > 0) ret += dfs(i - 1, j + 1, k) * i;
    if (j > 0) ret += dfs(i, j - 1, k + 1) * j;
    if (k > 0) ret += dfs(i, j, k - 1) * k;
    ret += N;
    ret /= sum;
    
    return dp[i][j][k] = ret;
}

int main() {
    // source code
    
    N = inputValue(); // 100
    vector<int> A(N); // 10 3以上は無視
    vector<int> R(3, 0); // 0枚のカード数, 1枚, 2枚
    rep(i, N){
        A[i] = inputValue();
        if (A[i] < 3) {
            R[A[i]]++;
        }
    }
    
    output(dfs(R[0], R[1], R[2]), 10);

    
    return 0;
}