#include "bits/stdc++.h"
using namespace std;

double dp[101][101][101];
int N;

double dfs(int a0, int a1, int a2){
	if (dp[a0][a1][a2] != 0) return dp[a0][a1][a2];
	int sum = a0 + a1 + a2;
	if (sum == 0) return 0;
	double ret = 0;
	if (a0) ret += dfs(a0 - 1, a1 + 1, a2) * a0;
	if (a1) ret += dfs(a0, a1 - 1, a2 + 1) * a1;
	if (a2) ret += dfs(a0, a1, a2 - 1) * a2;
	ret /= sum;
	ret += (double)N / sum;
	return dp[a0][a1][a2] = ret;
}

int main(){
	cin >> N;
	vector<int> A(N);
	for (int i = 0; i < N; i++)
	{
		cin >> A[i];
	}
	int num[3];
	num[0] = num[1] = num[2] = 0;
	for (int i = 0; i < N; i++)
	{
		if (A[i] < 3) num[A[i]]++;
	}
	double ans = dfs(num[0], num[1], num[2]);
	printf("%.14f\n", ans);
}