#include "bits/stdc++.h"
using namespace std;

typedef long long ll;
const int p = (int)1e9+7;

double dp[310][310][310];
int N;

double rec(int i, int j, int k) {
	if(dp[i][j][k] >= 0) return dp[i][j][k];
	if(i==0 && j==0 && k==0) return 0.0;

	double res=0.0;
	if(i>0) res += rec(i-1, j, k) * i;
	if(j>0) res += rec(i+1, j-1, k) * j;
	if(k>0) res += rec(i, j+1, k-1) * k;
	res += N;
	res *= 1.0 / (i+j+k);

	return dp[i][j][k] = res;
}

int main() {
	cin >> N;
	int one=0, two=0, three=0;
	for (int i = 0; i < N; i++) {
		int a; cin >> a;
		if(a==1) one++;
		else if(a==2) two++;
		else three++;
	}

	memset(dp, -1, sizeof(dp));

	cout << fixed << setprecision(10) << rec(one, two, three) << endl;
}