#include <cstdio>
using namespace std;

double dp[101][101][101];

int main() {
	int N; scanf("%d", &N);
	for (int k0 = 0; k0 <= N; k0++)
		for (int k1 = 0; k1 <= N; k1++)
			for (int k2 = 0; k2 <= N; k2++) {
				int k = k0 + k1 + k2;
				if (k == 0) continue;
				dp[k0][k1][k2] = (double)N / k;
				if (k0 > 0) dp[k0][k1][k2] += dp[k0 - 1][k1 + 1][k2] * k0 / k;
				if (k1 > 0) dp[k0][k1][k2] += dp[k0][k1 - 1][k2 + 1] * k1 / k;
				if (k2 > 0) dp[k0][k1][k2] += dp[k0][k1][k2 - 1] * k2 / k;
			}
	int k0 = 0, k1 = 0, k2 = 0;
	for (int i = 0; i < N; i++) {
		int A; scanf("%d", &A);
		if (A == 0) k0++;
		if (A == 1) k1++;
		if (A == 2) k2++;
	}
	printf("%.10f\n", dp[k0][k1][k2]);
}