#include <bits/stdc++.h>

using namespace std;

double dp[101][101][101];

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);

  int n;
  cin >> n;
  
  for (int i = 0; i <= 100; i++) {
    for (int j = 0; j <= 100; j++) {
      for (int k = 0; k <= 100; k++) {
        if (i + j + k == 0) continue;
        double sum = 1.0;
        if (i) sum += 1.0 * i / n * dp[i-1][j+1][k];
        if (j) sum += 1.0 * j / n * dp[i][j-1][k+1];
        if (k) sum += 1.0 * k / n * dp[i][j][k-1];
        sum *= 1.0 * n / (i + j + k);
        dp[i][j][k] = sum;
      }
    }
  }

  int rm[4] = {};
  for (int i = 0; i < n; i++) {
    int a;
    cin >> a;
    rm[max(3 - a, 0)]++;
  }

  cout << fixed << setprecision(12) << dp[rm[3]][rm[2]][rm[1]] << endl;
  
  return 0;
}