#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define Int long long

double dp[109][109][109];
int N;
double rec(int i, int j, int k) {
  if (i < 0 || j < 0 || k < 0) return 0LL;
  if (dp[i][j][k] != -1) return dp[i][j][k];
  double total = i + j + k;
  double res = N / total;
  res += rec(i - 1, j + 1, k) * i / total;
  res += rec(i, j - 1, k + 1) * j / total;
  res += rec(i, j, k - 1) * k / total;
  return dp[i][j][k] = res;
}

int main() {
  cin >> N;
  vector<int> cnt(3);
  rep(i, N) {
    int a;
    cin >> a;
    if (a < 3) cnt[a]++;
  }
  rep(i, 109) {
    rep(j, 109) { fill(dp[i][j], dp[i][j] + 109, -1); }
  }
  dp[0][0][0] = 0LL;
  cout << fixed << setprecision(7) << rec(cnt[0], cnt[1], cnt[2]) << endl;
}