#include <iostream>

using namespace std;

const int N = 110;
double dp[N][N][N];

int n;
double rec(int a, int b, int c){
	if (a + b + c == 0) return 0;

	double &res = dp[a][b][c];
	if (res >= -.5) return res;

	double dn = n;

	res = 1;
	if (a) res += a / dn*rec(a - 1, b + 1, c);
	if (b) res += b / dn*rec(a, b - 1, c + 1);
	if (c) res += c / dn*rec(a, b, c - 1);

	return res *= dn / (a + b + c);
}

int main(){
	cout.setf(ios::fixed);
	cout.precision(100);

	cin >> n;
	int cnt[3] = {};
	for (int i = 0; i < n; ++i){
		int x;
		cin >> x;
		if (x <= 2) ++cnt[x];
	}

	for (int i = 0; i < N; i++)
		for (int j = 0; j < N; j++)
			for (int k = 0; k < N; k++)
				dp[i][j][k] = -1;

	cout << rec(cnt[0], cnt[1], cnt[2]) << endl;
}