#include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <limits>
#include <climits>
#include <cfloat>
#include <functional>
using namespace std;

vector<vector<vector<double> > > memo;

double solve(int n, int a, int b, int c)
{
    if(a == 0 && b == 0 && c == 0)
        return 0.0;
    if(a < 0 || b < 0 || c < 0)
        return 0.0;
    if(memo[a][b][c] > -0.5)
        return memo[a][b][c];

    double ret = solve(n, a-1, b, c)   * a / n +
                 solve(n, a+1, b-1, c) * b / n +
                 solve(n, a, b+1, c-1) * c / n + 1;
    ret *= n / (double)(a + b + c);

    return memo[a][b][c] = ret;
}

int main()
{
    int n;
    cin >> n;

    vector<int> init(11);
    for(int i=0; i<n; ++i){
        int a;
        cin >> a;
        ++ init[a];
    }

    memo.assign(n+1, vector<vector<double> >(n+1, vector<double>(n+1, -1.0)));
    double ret = solve(n, init[2], init[1], init[0]);
    printf("%.10f\n", ret);

    return 0;
}