//#include <tourist>
#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
typedef long long ll;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef pair<ll, ll> p;
const int INF = 1e9;
const ll LINF = ll(1e18);
const int MOD = 1000000007;
const int dx[4] = {0, 1, 0, -1}, dy[4] = {-1, 0, 1, 0};
const int Dx[8] = {0, 1, 1, 1, 0, -1, -1, -1}, Dy[8] = {-1, -1, 0, 1, 1, 1, 0, -1};
#define yes cout << "Yes" << endl
#define YES cout << "YES" << endl
#define no cout << "No" << endl
#define NO cout << "NO" << endl
#define rep(i, n) for (int i = 0; i < n; i++)
#define ALL(v) v.begin(), v.end()
#define debug(v)          \
    cout << #v << ":";    \
    for (auto x : v)      \
    {                     \
        cout << x << ' '; \
    }                     \
    cout << endl;
template <class T>
bool chmax(T &a, const T &b)
{
    if (a < b)
    {
        a = b;
        return 1;
    }
    return 0;
}
template <class T>
bool chmin(T &a, const T &b)
{
    if (b < a)
    {
        a = b;
        return 1;
    }
    return 0;
}
//cout<<fixed<<setprecision(15);有効数字15桁
//-std=c++14
//g++ yarudake.cpp -std=c++17 -I .
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }
ll n;

double dfs(int i, int j, int k, vector<vector<vector<double>>> &memo)
{
    if(memo[i][j][k]>=0)return memo[i][j][k];
    //有効なカードがくるまで引く期待値は有効なカードが来る確率の逆数
    memo[i][j][k]=double(n)/(i+j+k);
    if(i){
        memo[i][j][k]+=dfs(i-1,j+1,k,memo)*i/(i+j+k);
    }
    if(j){
        memo[i][j][k] += dfs(i, j - 1, k+1,memo) * j / (i + j + k);
    }
    if (k)
    {
        memo[i][j][k] += dfs(i , j , k - 1,memo) * k / (i + j + k);
    }
    return memo[i][j][k];
}

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    cin >> n;
    vector<int> a(n);
    rep(i, n) cin >> a[i];
    vector<int> count(4, 0);
    rep(i, n)
    {
        if (a[i] <= 2)
            count[a[i]]++;
    }
    vector<vector<vector<double>>> memo(101, vector<vector<double>>(102, vector<double>(102, -1)));
    memo[0][0][0]=0;
    cout << fixed << setprecision(15);
    cout << dfs(count[0],count[1],count[2],memo) << "\n";
}