#include<bits/stdc++.h>
using namespace std;
#ifdef LOCAL_DEBUG
  #include "LOCAL_DEBUG.hpp"
#endif
#define int long long

template<class T> vector<T> make_vec(size_t a) { return vector<T>(a); }
template<class T, class... Ts> auto make_vec(size_t a, Ts... ts) {
  return vector<decltype(make_vec<T>(ts...))>(a, make_vec<T>(ts...));
}
template<class T, class V>
typename enable_if<is_class<T>::value == 0>::type fill(T &t, const V &v) {
    t = v;
}
template<class T, class V>
typename enable_if<is_class<T>::value != 0>::type fill(T &t, const V &v){
    for (auto &e : t) fill(e, v);
}
// auto v = make_vec<int>(h, w);
// fill(v, 0);

signed main(){

  int n; cin >> n;
  vector<int> a(n), cnt(4, 0);
  for(int i = 0; i < n; i++){
    cin >> a[i];
    cnt[min(3LL, a[i])]++;
  }

  auto dp = make_vec<double>(n+1, n+1, n+1);
  fill(dp, -1);
  dp[0][0][0] = 0;
  function< double(int,int,int) > rec =
  [&](int a, int b, int c){
    if(dp[a][b][c] != -1) return dp[a][b][c];
    double E = 1.0 * n / (a + b + c);
    if(a > 0) E += rec(a-1, b+1, c) * a / (a + b + c);
    if(b > 0) E += rec(a, b-1, c+1) * b / (a + b + c);
    if(c > 0) E += rec(a, b, c-1) * c / (a + b + c);
    return dp[a][b][c] = E;
  };
  printf("%.9f\n",rec(cnt[0], cnt[1], cnt[2]));

  return 0;
}