/* -*- coding: utf-8 -*-
 *
 * 108.cc: No.108 トリプルカードコンプ - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 100;

/* typedef */

/* global variables */

int n;
int n1 = 0, n2 = 0, n3 = 0;
double dp[MAX_N + 1][MAX_N + 1][MAX_N + 1];

/* subroutines */

double rec(int i3, int i2, int i1) {
  if (dp[i3][i2][i1] >= 0.0) return dp[i3][i2][i1];

  double sum = 0.0;
  if (i3 > 0) sum += rec(i3 - 1, i2 + 1, i1) * i3;
  if (i2 > 0) sum += rec(i3, i2 - 1, i1 + 1) * i2;
  if (i1 > 0) sum += rec(i3, i2, i1 - 1) * i1;

  return (dp[i3][i2][i1] = (sum + n) / (i3 + i2 + i1));
}

/* main */

int main() {
  cin >> n;

  for (int i = 0; i < n; i++) {
    int ai;
    cin >> ai;
    switch (ai) {
    case 0: n3++; break;
    case 1: n2++; break;
    case 2: n1++; break;
    }
  }

  for (int i3 = 0; i3 <= n; i3++)
    for (int i2 = 0; i2 <= n; i2++)
      for (int i1 = 0; i1 <= n; i1++) dp[i3][i2][i1] = -1.0;
  dp[0][0][0] = 0.0;

  double ans = rec(n3, n2, n1);
  printf("%.10lf\n", ans);

  return 0;
}