#define _USE_MATH_DEFINES
 
#include <iostream>
#include <iomanip>
#include <sstream>
 
#include <algorithm>
#include <cmath>
 
#include <string>
//#include <array>
#include <list>
#include <queue>
#include <vector>
#include <complex>
#include <set>
#include <map>
 
/////////
#define REP(i, x, n) for(int i = x; i < n; i++)
#define rep(i,n) REP(i,0,n)
#define P(p) cout<<(p)<<endl;
 
#define PII pair<int,int>
/////////
typedef long long LL;
typedef long double LD;
/////////
using namespace::std;
/////////

LD X[101][101][101];

int main(void){
    std::cin.tie(0); 
    std::ios::sync_with_stdio(false);
    std::cout << std::fixed;//
    cout << setprecision(16);//
	
	int N;
	cin>>N;
	int A[100];
	int a,b,c;
	a=b=c=0;

	rep(i,N){
		cin>>A[i];
		switch(A[i]){
		case 0:
			a++;
			break;
		case 1:
			b++;
			break;
		case 2:
			c++;
			break;
		}
	}
	LD sum,temp;
	for(int i=0;i<=N;++i){
		for(int j=0;j<=N;++j){
			for(int k=0;k<=N;++k){
				sum = i+j+k;
				if( sum ){
					temp = 0;
					if(i>0){
						temp += X[i-1][j+1][k] * i;
					}
					if(j>0){
						temp += X[i][j-1][k+1] * j;
					}
					if(k>0){
						temp += X[i][j][k-1] * k;
					}
					X[i][j][k] = (temp + N)/sum;
				}
			}
		}
	}
	P( X[a][b][c] );
	return 0;
}