#include <iostream>
#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
//#define _L(x) ;

template<int um> class UF {
public:
    vector<int> _parent,_rank;
    UF()
    {
        _parent=_rank=vector<int>(um,0);
        for(int i=0;i<um;i++)
        {
            _parent[i]=i;
        }
    }
    
    int Find(int x)
    {
        if(_parent[x] == x)
        {
            return x;
        }
        _parent[x] = Find(_parent[x]);
        return _parent[x];
    }
    
    int Union(int x,int y)
    {
        int xRoot = Find(x);
        int yRoot = Find(y);
        if(_rank[xRoot]>_rank[yRoot])
        {
            _parent[xRoot] = yRoot;
           return yRoot;
        }
        if(_rank[xRoot]<_rank[yRoot])
        {
            _parent[yRoot] = xRoot;
            return xRoot;
        }
        if(xRoot != yRoot)
        {
            _parent[yRoot] = xRoot;
            _rank[xRoot]++;
            return xRoot;
        }
        return xRoot;
    }
};

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