#include <iostream>
#include <fstream>
#include <iomanip>
#include <climits>
#include <limits>
#include <algorithm>
#include <array>
#include <vector>
#include <deque>
#include <queue>
#include <list>
#include <stack>
#include <string>
#include <functional>
#include <numeric>
#include <map>
#include <set>
#include <cstdlib>
#include <bitset>
#include <unordered_map>
#include <random>
#include <cmath>
#include <complex>
// #include "utiltime.hpp"

using namespace std;

typedef long long int ll;
typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef pair<int, int> P;
typedef pair<ll, ll> Pll;
typedef vector<ll> vll;
typedef vector<vector<ll>> vvll;
typedef complex<double> cdouble;

const int INFL = (int)1e9;
const ll INFLL = (ll)1e18;
const double INFD = numeric_limits<double>::infinity();
const double PI = 3.14159265358979323846;
#define Loop(i, n) for(int i = 0; i < (int)(n); i++)
#define Loopll(i, n) for(ll i = 0; i < (ll)(n); i++)
#define Loop1(i, n) for(int i = 1; i <= (int)(n); i++)
#define Loopll1(i, n) for(ll i = 1; i <= (ll)(n); i++)
#define Loopr(i, n) for(int i = (int)(n) - 1; i >= 0; i--)
#define Looprll(i, n) for(ll i = (ll)(n) - 1; i >= 0; i--)
#define Loopr1(i, n) for(int i = (int)(n); i >= 1; i--)
#define Looprll1(i, n) for(ll i = (ll)(n); i >= 1; i--)
#define Foreach(buf, container) for(auto buf : container)
#define Loopdiag(i, j, h, w, sum) for(int i = ((sum) >= (h) ? (h) - 1 : (sum)), j = (sum) - i; i >= 0 && j < (w); i--, j++)
#define Loopdiagr(i, j, h, w, sum) for(int j = ((sum) >= (w) ? (w) - 1 : (sum)), i = (sum) - j; j >= 0 && i < (h); j--, i++)
#define Loopdiagsym(i, j, h, w, gap) for (int i = ((gap) >= 0 ? (gap) : 0), j = i - (gap); i < (h) && j < (w); i++, j++)
#define Loopdiagsymr(i, j, h, w, gap) for (int i = ((gap) > (h) - (w) - 1 ? (h) - 1 : (w) - 1 + (gap)), j = i - (gap); i >= 0 && j >= 0; i--, j--)
#define Loopitr(itr, container) for(auto itr = container.begin(); itr != container.end(); itr++)
#define printv(vector) Loop(extra_i, vector.size()) { cout << vector[extra_i] << " "; } cout << endl;
#define printmx(matrix) Loop(extra_i, matrix.size()) { Loop(extra_j, matrix[extra_i].size()) { cout << matrix[extra_i][extra_j] << " "; } cout << endl; }
#define quickio() ios::sync_with_stdio(false); cin.tie(0);
#define readfile(filename) ifstream in(filename); cin.rdbuf(in.rdbuf());
#define bitmanip(m,val) static_cast<bitset<(int)m>>(val)
bool nearlyeq(double x, double y) { return abs(x - y) < 1e-9; }
bool inrange(int x, int t) { return x >= 0 && x < t; }
ll rndf(double x) { return (ll)(x + (x >= 0 ? 0.5 : -0.5)); }
ll floorsqrt(double x) { ll m = (ll)sqrt(x); return m + (m * m <= (ll)(x) ? 0 : -1); }
ll ceilsqrt(double x) { ll m = (ll)sqrt(x); return m + ((ll)x <= m * m ? 0 : 1); }
ll rnddiv(ll a, ll b) { return (a / b + (a % b * 2 >= b ? 1 : 0)); }
ll ceildiv(ll a, ll b) { return (a / b + (a % b == 0 ? 0 : 1)); }
ll gcd(ll m, ll n) { if (n == 0) return m; else return gcd(n, m % n); }

/*******************************************************/

vector<vector<vector<double>>> dp;
vector<vector<vector<double>>> p;
int n;
vector<double> invs;
queue<vi> que;

void calc(int xi, int xj, int xk) {
	dp[xi][xj][xk] /= p[xi][xj][xk];
	int sum = xi + xj + xk;
	double buf = dp[xi][xj][xk] + double(n) * invs[sum];
	if (inrange(xi - 1, n + 1)) {
		double pbuf = p[xi][xj][xk] * invs[sum] * xi;
		dp[xi - 1][xj][xk] += buf * pbuf;
		p[xi - 1][xj][xk] += pbuf;
		que.push({ xi - 1, xj, xk });
	}
	if (inrange(xi + 1, n + 1) && inrange(xj - 1, n + 1)) {
		double pbuf = p[xi][xj][xk] * invs[sum] * xj;
		dp[xi + 1][xj - 1][xk] += buf * pbuf;
		p[xi + 1][xj - 1][xk] += pbuf;
		que.push({ xi + 1, xj - 1, xk });
	}
	if (inrange(xj + 1, n + 1) && inrange(xk - 1, n + 1)) {
		double pbuf = p[xi][xj][xk] * invs[sum] * xk;
		dp[xi][xj + 1][xk - 1] += buf * pbuf;
		p[xi][xj + 1][xk - 1] += pbuf;
		que.push({ xi, xj + 1, xk - 1 });
	}
}

int main() {
	cin >> n;
	vi cnt(3);
	Loop(i, n) {
		int x; cin >> x;
		cnt[x - 1]++;
	}
	invs = vector<double>(301);
	Loop1(i, 300) invs[i] = 1.0 / i;
	dp = vector<vector<vector<double>>>(n + 1, vector<vector<double>>(n + 1, vector<double>(n + 1)));
	p = vector<vector<vector<double>>>(n + 1, vector<vector<double>>(n + 1, vector<double>(n + 1)));
	p[cnt[0]][cnt[1]][cnt[2]] = 1;
	que.push({ cnt[0], cnt[1], cnt[2] });
	vector<vector<vector<bool>>> done(n + 1, vector<vector<bool>>(n + 1, vector<bool>(n + 1, false)));
	while (que.size()) {
		if (!done[que.front()[0]][que.front()[1]][que.front()[2]]) {
			calc(que.front()[0], que.front()[1], que.front()[2]);
			done[que.front()[0]][que.front()[1]][que.front()[2]] = true;
		}
		que.pop();
	}
	cout << setprecision(15) << dp[0][0][0] << endl;
}

Measured memory_bytes: 441728