#include<bits/stdc++.h>
using namespace std;
const int N=305;
double dp[N][N][N];
int n,a,b,c,x;
double dfs(int A,int B,int C)
{
	if(!A&&!B&&!C)return 0;
	if(dp[A][B][C]!=0.00)return dp[A][B][C];
	double sum=1;
	if(A)
	{
		sum+=(double)A*dfs(A-1,B,C)/(double)n;
	}
	if(B)
	{
		sum+=(double)B*dfs(A+1,B-1,C)/(double)n;
	}
	if(C)
	{
		sum+=(double)C*dfs(A,B+1,C-1)/(double)n;
	}
	sum+=sum/(double)(A+B+C)*(double)(n-A-B-C);
	return dp[A][B][C]=sum;
}
int main()
{
	cin>>n;
	for(int i=0;i<n;i++)
	{
		cin>>x;if(x==1)a++;else if(x==2)b++;else c++;
	}
	double ans=dfs(a,b,c);
	printf("%.13f\n",ans);
	return 0;
}