#define __mode_debug__
/*   ξ
     ll
   ＿ll＿
 ／  ∞  ＼
│*  Ａ  *│
│*  Ｃ  *│
│*  祈  *│
│*  願  *│
│*      *│
 ￣￣￣￣￣    */
//C++14 (GCC 5.4.1)
#include <bits/stdc++.h>
using namespace std;
#ifdef __mode_debug__
    #define DBG
#else
    #define DBG if(false)
#endif

#define LLINF (1LL<<60)
using ll = long long;
#define mod  1000000007

#define whole(a) (a).begin(),(a).end()
#define rwhole(a) (a).rbegin(),(a).rend()

#define repd(i,a,b) for (int i=(a);i<(b);i++)
#define rep(i,n) repd(i,0,n)
#define rrepd(i,a,b) for(int i=(a);i>=(b);i--)
#define each(itr,ds) for(auto itr=(ds).begin();itr!=(ds).end();itr++)

template<typename T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<typename T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

template < typename T > inline string toString( const T &a ) { ostringstream oss; oss << a; return oss.str(); }

//DUMP
//  ostream vector<T> , vector<vector<T>> , set<T> ,queue<T> , priority_queue<T> , pair<T,U> , map<T,U>
template<class T> ostream& operator<<(ostream&os,vector<T>&vec){os<<"{ ";rep(i,vec.size()) os<<((i==0)?"":", ")<<i<<":"<<vec[i];os<<" }";return os;}
template<class T> ostream& operator<<(ostream&os,vector<vector<T>>&vec){os<<"{"<<endl;rep(i,vec.size())os<<"\t   "<<i<<":"<<vec[i]<<((i==vec.size()-1)? "":",")<<endl;os<<"\t }";return os;}
template<class T> ostream& operator<<(ostream&os,set<T>&p){os<<"{ ";each(itr,p)  os<<((itr==p.begin())?"":", ")<<*itr;os<<" }";return os;}
template<class T> ostream& operator<<(ostream&os,queue<T>&p){queue<T> q(p);os<<"[< "<<(q.empty()?" <]":"");while(!q.empty()){os<<q.front();q.pop();os<< (q.empty() ? " <]" : ", ");}return os;}
template<class T> ostream& operator<<(ostream&os,priority_queue<T>&p){priority_queue<T> q(p);os<<"[< "<<(q.empty()?" <]":"");while(!q.empty()){os<<q.top();q.pop();os<< (q.empty() ? " <]" : ", ");}return os;}
template<class T,class U> ostream& operator<<(ostream&os,pair<T,U>&p){os<<"("<<p.first<<","<<p.second<<")";return os;}
template<class T,class U> ostream& operator<<(ostream&os,map<T,U>&p){os<<"{ ";each(itr,p)os<<((itr==p.begin())?"":", ")<<itr->first<<":"<<itr->second;os<<" }";return os;}
void dump_f(string &nm,char t){cerr<<endl;}
template<class T,class... Ts> void dump_f(string &nm,char t,T &&x,Ts&&... ts){int l=nm.find_first_of(',');string name(nm,0,l),nx(nm,l+1,-1);if(t=='\n') cerr<<"  "<<name<<"\t: "<<x<<t;else cerr << name<<": "<<x<<","<<t;dump_f(nx,t,forward<Ts>(ts)...);}
template<class... Ts> void dump_m(int &&ln,string &&nm,char t,Ts&&... ts){cerr<<"[ln: "<<ln<<"]"<<t;dump_f(nm,t,forward<Ts>(ts)...);}
#define dump(...) DBG dump_m(__LINE__,#__VA_ARGS__,'\n',__VA_ARGS__)
#define dl(...) DBG dump_m(__LINE__,#__VA_ARGS__,'\t',__VA_ARGS__)

#define lower_bound_idx(V,c) distance(V.begin(),lower_bound(whole(V),c))

int N;
double memo[301][301][301]={0};
double howeat(int s1,int s2,int s3){
    if(memo[s1][s2][s3]!=0) return memo[s1][s2][s3];
    if(s1==0&&s2==0&&s3==0) return 0.0;
    int sn=s1+s2+s3;
    double x=0.0,Nsn=(double)N/sn;
    if(s3>0)x+=(howeat(s1  ,s2+1,s3-1)+Nsn)*s3/sn;
    if(s2>0)x+=(howeat(s1+1,s2-1,s3  )+Nsn)*s2/sn;
    if(s1>0)x+=(howeat(s1-1,s2  ,s3  )+Nsn)*s1/sn;
    memo[s1][s2][s3]=x;
    //dl(Nsn,s1,s2,s3,x);
    return x;
}

/*double howeat(vector<int> &x){
    if(memo[x]!=0.0) return memo[x];
    if(x[3]==0&&x[2]==0&&x[1]==0) return 0;
    double nxt=0.0,de,pr, Q=N-x[0];
    for(int i=3;i>0;i--){
        if(x[i]==0)continue;
        de=(double)N/Q;
        pr=x[i]/Q;
        x[i]--;
        x[i-1]++;
        nxt+=(howeat(x)+de)*pr;
        x[i-1]--;
        x[i]++;
    }
    memo[x]=nxt;
    //dl(x,nxt);
    return nxt;
}*/


int main() {
    ios::sync_with_stdio(false);//stdout<=>cout
    cin.tie(nullptr);//cin<=>cout

    cin >>N;
    int x;
    vector<int> sushi(4,0);
    rep(i,N){
        cin>>x;
        sushi[x]++;
    }
    double ans=howeat(sushi[1],sushi[2],sushi[3]);
    
    printf("%.14f",ans);

    return (0);
}

Measured memory_bytes: 213248