// includes
#include <bits/stdc++.h>

// macros
#define ll long long int
#define pb emplace_back
#define mk make_pair
#define pq priority_queue
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define rep(i, n) FOR(i, 0, n)
#define rrep(i, n) for(int i=((int)(n)-1);i>=0;i--)
#define all(x) (x).begin(),(x).end()
#define sz(x) ((int)(x).size())
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end())
#define FI first
#define SE second
using namespace std;

//  types
typedef pair<int, int> P;
typedef pair<ll, int> Pl;
typedef pair<ll, ll> Pll;
typedef pair<double, double> Pd;
 
// constants
const int inf = 1e9;
const ll linf = 1LL << 50;
const double EPS = 1e-10;
const int mod = 1e9 + 7;

// solve
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(a > b){a = b; return 1;} return 0;}
template <typename T> istream &operator>>(istream &is, vector<T> &vec){for(auto &v: vec)is >> v; return is;}

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

double calc(int i, int j, int k){
  if(dp[i][j][k] + EPS >= 0.)return dp[i][j][k];
  double res = n;
  if(i > 0)res += (double)i * calc(i-1, j+1, k);
  if(j > 0)res += (double)j * calc(i, j-1, k+1);
  if(k > 0)res += (double)k * calc(i, j, k-1);
  res = res / (double)(i + j + k);
  return dp[i][j][k] = res;
}

int main(int argc, char const* argv[])
{
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cin >> n;
  vector<int> a(n);
  cin >> a;
  int I = 0, J = 0, K = 0;
  rep(i, n){
    if(a[i] == 0)I++;
    else if(a[i] == 1)J++;
    else if(a[i] == 2)K++;
  }
  rep(i, 101){
    rep(j, 101){
      rep(k, 101)dp[i][j][k] = -1.;
    }
  }
  dp[0][0][0] = 0.;
  cout << setprecision(20);
  cout << calc(I, J, K) << endl;
	return 0;
}