#include<iostream>
#include<vector>
#include<map>
#include<numeric>
#include<cstdio>
using namespace std;

int nType, totalCard;
vector<int> init;

void read() {
    init = vector<int>(4, 0);
    
    cin >> nType;
    for (int i = 0; i < nType; ++i) {
        int cnt;
        cin >> cnt;
        ++init[min(3, cnt)];
    }
    
    totalCard = accumulate(init.begin(), init.end(), 0);
}


double rec(vector<int> &num2cnt, map<vector<int>, double> &dp) {
    if (num2cnt[3] == nType) return 0;
    if (dp.count(num2cnt)) return dp[num2cnt];
    double sum = 1;
    
    for (int i = 0; i < 3; ++i) {
        if (num2cnt[i] > 0) {
            double mul = 1.0 * num2cnt[i] / totalCard;
            --num2cnt[i];
            ++num2cnt[i + 1];
            sum += rec(num2cnt, dp) * mul;
            ++num2cnt[i];
            --num2cnt[i + 1];
        }
    }

    return dp[num2cnt] = sum / (1 - 1.0 * num2cnt[3] / totalCard);
}


void work() {
    map<vector<int>, double> dp;
    printf("%.10lf\n", rec(init, dp));
}


int main() {
    read();
    work();
    return 0;
}