#include <bits/stdc++.h>
// #include <atcoder/modint>
#define rng(a) a.begin(),a.end()
#define rrng(a) a.rbegin(),a.rend()
#define INF 2000000000000000000
#define ll long long
#define ld long double
#define pll pair<ll, ll>
using namespace std;
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; }

ll N;
map<vector<ll>, ld> mp;
ld bfs(ll x, ll y, ll z) {
  if (mp.find({x, y, z}) != mp.end()) {
    return mp[{x, y, z}];
  }
  if (z == N) {
    return 0;
  }
  ld res = 0;
  ld mul = (N - z) / (ld)(N);
  if (x + y + z != N) {
    res += (bfs(x + 1, y, z) * (N - (x + y + z)) / (ld)(N));
  }
  if (x != 0) {
    res += ((bfs(x - 1, y + 1, z)) * (x) / (ld)(N));
  }
  if (y != 0) {
    res += ((bfs(x, y - 1, z + 1)) * (y) / (ld)(N));
  }
  mp[{x, y, z}] = (res + 1) / mul;
  return (res + 1) / mul;
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cin >> N;
  vector<ll> A(N);
  vector<ll> num(4, 0);
  for (ll i = 0; i < N; ++i) {
    cin >> A.at(i);
    num.at(min(3ll, A.at(i))) += 1;
  }
  cout << fixed << setprecision(15);
  cout << bfs(num.at(1), num.at(2), num.at(3)) << "\n";
}