#include <iostream>
#include <cstdio>
#include <fstream>
#include <algorithm>
#include <string>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <vector>
#include <limits.h>
#include <math.h>
#include <functional>
#include <bitset>
#include <iomanip>
#include <cassert>
#include <float.h>
#include <random>

#define repeat(i,n) for (int i = 0; (i) < (n); ++ (i))
#define debug(x) cerr << #x << ": " << x << '\n'
#define debugArray(x,n) for(long long hoge = 0; (hoge) < (n); ++ (hoge)) cerr << #x << "[" << hoge << "]: " << x[hoge] << '\n'
#define debugArrayP(x,n) for(long long hoge = 0; (hoge) < (n); ++ (hoge)) cerr << #x << "[" << hoge << "]: " << x[hoge].first<< " " << x[hoge].second << '\n'

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> Pii;
typedef vector<int> vint;
typedef vector<ll> vll;
const ll INF = LLONG_MAX/10;
const ll MOD = 1e9+7;

int N;
double dp[110][110][110];
int main(){
  cin.tie(0);
  ios::sync_with_stdio(false);
  cin>>N;
  int cnt[3]={0};
  repeat(i,N){
    int A;cin>>A;
    cnt[2] += A>=3;
    cnt[1] += A>=2;
    cnt[0] += A>=1;
  }
  for(int i=N;i>=0;i--){
    for(int j=i;j>=0;j--){
      for(int k=min(j,N-1);k>=0;k--){
        dp[i][j][k] = dp[i+1][j][k]*(N-i)/(N-k)+dp[i][j+1][k]*(i-j)/(N-k)+dp[i][j][k+1]*(j-k)/(N-k)+(double)N/(N-k);
      }
    }
  }
  //debugArray(cnt,3);
  printf("%.7lf\n",dp[cnt[0]][cnt[1]][cnt[2]]);
  return 0;
}