#include <bits/stdc++.h>
using namespace std;
using ld = long double;

int main()
{
	cout << fixed << setprecision(8);
	int N;
	cin >> N;
	int X = 0, Y = 0, Z = 0;
	for (int i = 0; i < N; i++) {
		int A;
		cin >> A;
		if (A == 0) ++X;
		if (A == 1) ++Y;
		if (A == 2) ++Z;
	}
	vector<vector<vector<ld>>> dp(N + 1, vector<vector<ld>>(N + 1, vector<ld>(N + 1)));
	for (int i = 0; i <= N; i++) {
		for (int j = 0; j <= N; j++) {
			for (int k = 0; k <= N; k++) if (i + j + k <= N) {
				if (0 <= i - 1) {
					dp[i][j][k] += (dp[i - 1][j + 1][k] + (ld)N / (i + j + k)) * ((ld)i / (i + j + k));
				}
				if (0 <= j - 1) {
					dp[i][j][k] += (dp[i][j - 1][k + 1] + (ld)N / (i + j + k)) * ((ld)j / (i + j + k));
				}
				if (0 <= k - 1) {
					dp[i][j][k] += (dp[i][j][k - 1] + (ld)N / (i + j + k)) * ((ld)k / (i + j + k));
				}
			}
		}
	}
	cout << dp[X][Y][Z] << endl;
	return 0;
}