#include "bits/stdc++.h"
using namespace std;
#define int long long
#define REP(i, n) for (int i = 0; i < (int)n; ++i)
#define RREP(i, n) for (int i = (int)n - 1; i >= 0; --i)
#define FOR(i, s, n) for (int i = s; i < (int)n; ++i)
#define RFOR(i, s, n) for (int i = (int)n - 1; i >= s; --i)
#define ALL(a) a.begin(), a.end()
#define IN(a, x, b) (a <= x && x < b)
template<class T>inline void out(T t){cout << t << "\n";}
template<class T,class... Ts>inline void out(T t,Ts... ts){cout << t << " ";out(ts...);}
template<class T>inline bool CHMIN(T&a,T b){if(a > b){a = b;return true;}return false;}
template<class T>inline bool CHMAX(T&a,T b){if(a < b){a = b;return true;}return false;}
constexpr int INF = 1e18;

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

signed main(){
	int N;
	cin >> N;
	vector<int>a(N), cnt(4);
	REP(i, N) {
		cin >> a[i];
		cnt[min(3ll,a[i])]++;
	}
	memset(dp, -1, sizeof(dp));
	dp[0][0][0] = 0;
	auto dfs = [&](auto&&f, int a, int b, int c) -> double{
		if(dp[a][b][c] > -0.5) return dp[a][b][c];
		int d = N - a - b - c;
		dp[a][b][c] = (double) N / (a + b + c);
		if(a)dp[a][b][c] += f(f, a - 1, b, c) * a / (N - d);
		if(b)dp[a][b][c] += f(f, a + 1, b - 1, c) * b / (N - d);
		if(c)dp[a][b][c] += f(f, a, b + 1, c - 1) * c / (N - d);
		return dp[a][b][c];
	};
	cout << setprecision(20) << dfs(dfs, cnt[2], cnt[1], cnt[0]) << endl;
}