#include <bits/stdc++.h>

using namespace std;

int N;
double memo[128][128][128];

double calc(int zero, int one, int two)
{
	if (zero + one + two == 0) return (0);
	if (memo[zero][one][two] >= 0) return (memo[zero][one][two]);

	double ret = 1;
	if (zero) ret += zero * 1. / N * calc(zero - 1, one + 1, two);
	if (one)  ret += one  * 1. / N * calc(zero, one - 1, two + 1);
	if (two)  ret += two  * 1. / N * calc(zero, one, two - 1);
	
	ret *= N * 1. / (zero + one + two);

	return (memo[zero][one][two] = ret);
}

int main()
{
	int ct[3] = {0};

	scanf("%d", &N);

	for (int i = 0; i < N; i++){
		int x;
		scanf("%d", &x);
		if (x < 3) ct[x]++;
	}

	for (int i = 0; i < 128; i++) for (int j = 0; j < 128; j++) for (int k = 0; k < 128; k++) memo[i][j][k] = -1;

	printf("%.10lf\n", calc(ct[0], ct[1], ct[2]));

	return (0);
}