#include "bits/stdc++.h"
#define all(x) (x).begin(), (x).end()
#define rep(i,a,b) for (int (i) = (a); (i) < (b); ++(i))
#define rrep(i,a,b) for (int (i) = (a); (i) >= (b); --(i))
using namespace std;
typedef long long ll;
const int INF = 1e9;
const ll LINF = 1LL << 60;
const ll MOD = 1e9 + 7;

template<class T> bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }

int N;
int c0, c1, c2;
double dp[110][110][110];

double dfs(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;

	double res = N;
	if (i > 0) res += dfs(i - 1, j + 1, k) * i;
	if (j > 0) res += dfs(i, j - 1, k + 1) * j;
	if (k > 0) res += dfs(i, j, k - 1) * k;
	res /= (double)(i + j + k);
	return dp[i][j][k] = res;
}

int main() {
	cin >> N;
	rep(i, 0, N) {
		int a;
		cin >> a;
		if (a == 0) ++c0;
		if (a == 1) ++c1;
		if (a == 2) ++c2;
	}

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

	cout << dfs(c0, c1, c2) << endl;
	return 0;
}