/* @Author: Shubham Anand */

#include <unordered_map>
#include <algorithm>
#include <iostream>
#include <iomanip>
#include <limits>
#include <chrono>
#include <vector>
#include <cmath>
#include <queue>
#include <stack>
#include <list>
#include <map>
#include <set>

#define FILEIN          freopen("input.txt", "r", stdin)
#define FILEOUT         freopen("output.txt", "w", stdout)
#define FASTIO          std::ios_base::sync_with_stdio(0); std::cin.tie(NULL); std::cout.tie(NULL)
#define PAUSE           while(1){}
#define NEWLINE         std::cout << "\n"
#define EDGES           std::unordered_map<std::pair<int,int>,int>
#define PB(N)           push_back(N)

/* Constants */
const unsigned int MODULO = 1e9+7;
const unsigned int INF = std::numeric_limits<int>::max();
const int NINF = std::numeric_limits<int>::min();

/* Input Output */
template <class T> void write(const T &a, char del='\n')        { if(del=='~')std::cout<<a; else std::cout<<a<<del; }
template <class T> void line(const T &w)                        { for(int i=0;i<(w+1)/2;i++)write('_','_');NEWLINE; }
template <class T> void read(T &a)                              { std::cin >> a; }

/* Modular Ops */
template <class T> T add(T a, T b, T m) { a=a%m; b=b%m; return (a+b)%m; }
template <class T> T mul(T a, T b, T m) { a=a%m; b=b%m; return (a*b)%m; }

/* Basic Algorithms */
template <class T> T gcd(T a, T b)      { if(b>a){return gcd(b,a);} if(b==0){return a;} return gcd(b, a%b); }
template <class T> T bexp(T a, T b, T m){ if(b==1){return a;}if(b&1){return mul(a,bexp(a,b-1,m),m);}T c=bexp(a,b/2,m); return mul(c,c,m);}
template <class T> T inv(T a, T m)      { return bexp(a,m-2,m); }

/* Commonly used data structures */
namespace ds {
    template<class T> struct Point{ T x,y,z; Point(T _x=0,T _y=0,T _z=0):x(_x), y(_y), z(_z){}; };
    template<class T> struct Graph{ std::vector<T> *adj; Graph(T n){adj=new std::vector<T>[n];} void add(T u, T v, bool d=0){adj[u].PB(v);if(!d){adj[v].PB(u);}} void w_add(T u, T v, T w, bool d=0){adj[u].PB(std::make_pair(v,w)); if (!d){adj[v].PB(std::make_pair(u,w));}}};
}

/* Function to calculate runtime */
void time(void func()) {
    std::chrono::time_point<std::chrono::high_resolution_clock> start, end;
    start = std::chrono::high_resolution_clock::now();
    func();
    end = std::chrono::high_resolution_clock::now();
    long long elapsed_time = std::chrono::duration_cast<std::chrono::milliseconds>(end-start).count();
    std::cout << "\tElapsed Time: " << elapsed_time << "ms\n";
}

/* Declare any global objects here */
const int nax = 302;
double cache[nax][nax][nax];
double Q;
/************** END ****************/

double dfs(int a, int b, int c) {
    
    if (cache[a][b][c]>=0) {
        return cache[a][b][c];
    }
    if (a==0 && b==0 && c==0) {
        return 0;
    }
    double node_expected = Q/(a+b+c);
    double expected = 0;
    if (a) { expected += (node_expected + dfs(a-1, b, c))*(double)a/(a+b+c); }
    if (b) { expected += (node_expected + dfs(a+1, b-1, c))*(double)b/(a+b+c); }
    if (c) { expected += (node_expected + dfs(a, b+1, c-1))*(double)c/(a+b+c); }

    cache[a][b][c] = expected;
    return expected;

}

void solve() {
    
    int N; read(N);

    int cnt[3]; for (int i=0; i<3; i++) { cnt[i]=0; }
    int num;

    for (int i=0; i<N; i++) { read(num); cnt[num-1]++; }

    Q = cnt[0] + cnt[1] + cnt[2];  
    for (int i=0; i<nax; i++) { for (int j=0; j<nax; j++) { for (int k=0; k<nax; k++) { cache[i][j][k]=-1; } } }
    std::cout << std::fixed << std::setprecision(10) << dfs(cnt[0], cnt[1], cnt[2]) << "\n";

}

signed main() {

    // FILEIN; 
    // FILEOUT;
    FASTIO;

    int T = 1;
    // Uncomment to read testcases
    // read(T);
    
    while (T--) {
        // time(solve);
        solve();
    }

    return 0;
}

Measured memory_bytes: 215552