#include<bits/stdc++.h>
using namespace std;
using Int = long long;
template<typename T1,typename T2> inline void chmin(T1 &a,T2 b){if(a>b) a=b;}
template<typename T1,typename T2> inline void chmax(T1 &a,T2 b){if(a<b) a=b;}


struct Precision{
  Precision(){
    cout<<fixed<<setprecision(12);
  }
}precision_beet;

//INSERT ABOVE HERE
double dp[111][111][111];
signed main(){
  Int n;
  cin>>n;
  vector<Int> a(n);
  for(Int i=0;i<n;i++) cin>>a[i];

  for(Int i=0;i<111;i++)
    for(Int j=0;j<111;j++)
      for(Int k=0;k<111;k++)
        dp[i][j][k]=-1;
  function<double(Int, Int, Int)> dfs=
    [&](Int x,Int y,Int z)->double{
      double &res=dp[x][y][z];
      if(res>=0) return res;
      if(x==0&&y==0&&z==0) return res=0;
      res=1;
      if(x) res+=dfs(x-1,y+1,z)*x/n;
      if(y) res+=dfs(x,y-1,z+1)*y/n;
      if(z) res+=dfs(x,y,z-1)*z/n;
      res*=1.0*n/(x+y+z);
      return res;
    };
  Int x=0,y=0,z=0;
  for(Int i=0;i<n;i++){
    if(a[i]==0) x++;
    if(a[i]==1) y++;
    if(a[i]==2) z++;
  }
  cout<<dfs(x,y,z)<<endl;
  return 0;
}