#include <bits/stdc++.h>

using namespace std;

int n;
double dp[105][105][105];

double solve(int cnt0, int cnt1, int cnt2) {
	if (cnt0 + cnt1 + cnt2 == 0) return 0;
	if (dp[cnt0][cnt1][cnt2] != -1) return dp[cnt0][cnt1][cnt2];
	double res = n - cnt0 - cnt1 - cnt2;
	if (cnt0 > 0) res += cnt0 * (solve(cnt0 - 1, cnt1 + 1, cnt2) + 1);
	if (cnt1 > 0) res += cnt1 * (solve(cnt0, cnt1 - 1, cnt2 + 1) + 1);
	if (cnt2 > 0) res += cnt2 * (solve(cnt0, cnt1, cnt2 - 1) + 1);
	res /= cnt0 + cnt1 + cnt2;
	return dp[cnt0][cnt1][cnt2] = res;
}

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	cin >> n;
	int cnt[3] = {};
	for (int i = 0; i < n; i++) {
		int a;
		cin >> a;
		if (a < 3) cnt[a]++;
	}
	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 << fixed << setprecision(10) << solve(cnt[0], cnt[1], cnt[2]) << endl;
	return 0;
}