#include <iostream>
#include <iomanip>
#include <vector>
#include <list>
#include <deque>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <algorithm>
#include <cmath>
#include <cstring>
#include <limits>

using namespace std;

#define FOR(x,y) for(int x = 0;x < (y);x++)
#define LLI long long int
#define FORR(x,arr) for(auto& x:arr)
#define ALL(a) (a.begin()),(a.end())

#define _L(x) cout<<(x)<<endl

int n;
double dp[101][101][101];

double solve(int a, int b, int c)
{
    if(dp[a][b][c] >= 0)
    {
        return dp[a][b][c];
    }
    double div = a + b + c;
    double expect = (double)n / div;
    if(c > 0)
    {
        expect += (double)c / div * solve(a,b,c-1);
    }
    if(b > 0)
    {
        expect += (double)b / div * solve(a,b-1,c+1);
    }
    if(a > 0)
    {
        expect += (double)a / div * solve(a-1,b+1,c);
    }
    dp[a][b][c] = expect;
    //cout << a << "," << b << "," << c << "," << expect << endl;
    return expect;
}

int main()
{
    cin >> n;
    vector<int> data(n);
    int a = 0;
    int b = 0;
    int c = 0;
    FOR(i, n)
    {
        cin >> data[i];
        if(data[i] == 0)
        {
            a++;
        }
        else if(data[i] == 1)
        {
            b++;
        }
        else if(data[i] == 2)
        {
            c++;
        }
    }
    FOR(i, 101) FOR(j, 101) FOR(k, 101)
    {
        dp[i][j][k] = -1;
    }
    dp[0][0][0] = 0;
    cout << fixed << setprecision(12) << solve(a,b,c) << endl;
    return 0;
}