#include <iostream>
#include <cmath>
#include <vector>
#include <utility>
#include <queue>
#include <map>
#include <string>
#include <cstring>
#include <list>
#include <algorithm>
#define INF 114514

using namespace std;

typedef pair<double,double> P;
int n;

double memo[101][101][101];
double dfs(int n3,int n2,int n1){
	if(memo[n3][n2][n1] >= 0)return memo[n3][n2][n1];

	double ret = n*1.0/(n1+n2+n3);
	if(n1>0)ret += dfs(n3,n2,n1-1) * n1/(n1+n2+n3);
	if(n2>0)ret += dfs(n3,n2-1,n1+1) * n2/(n1+n2+n3);
	if(n3>0)ret += dfs(n3-1,n2+1,n1) * n3/(n1+n2+n3);
	return memo[n3][n2][n1] = ret;
}

int main(void){
	cin >> n;
	vector<int> a(n);
	for(int i=0;i<n;i++)
		cin >> a[i];
	
	int n3,n2,n1;
	n3=n2=n1=0;
	for(int i=0;i<n;i++){
		if(a[i] == 0)n3++;
		if(a[i] == 1)n2++;
		if(a[i] == 2)n1++;
	}
	memset(memo,-1,sizeof(memo));
	memo[0][0][0] = 0;
	printf("%.12lf\n",dfs(n3,n2,n1));

	return 0;
}