#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000

int n;
double dp[102][102][102];
bool f[102][102][102];

double get(int a,int b,int c){
	if(a==0&&b==0&&c==0)return 0.0;
	int d = n - a - b - c;
	if(f[a][b][c])return dp[a][b][c];
	f[a][b][c] = true;
	double ret = 0.0;
	if(a!=0){
		ret += get(a-1,b+1,c) * a / (double)n;
	}
	if(b!=0){
		ret += get(a,b-1,c+1) * b / (double)n;
	}
	if(c!=0){
		ret += get(a,b,c-1) * c / (double)n;
	}
	ret += 1.0;
	ret /= 1-(double)d/(double)n;
	
	dp[a][b][c] = ret;
	return ret;
	
}

int main(){
	
	cin>>n;
	
	vector<int> c(3,0);
	rep(i,n){
		int a;
		cin>>a;
		if(a<3)c[a]++;
	}
	
	cout<<fixed<<setprecision(10)<<get(c[0],c[1],c[2])<<endl;
	
	return 0;
}