#include "bits/stdc++.h"

using ll = long long;

using namespace std;
#define int ll

//0 -> a-1
#define rep(i, a) for (int i = 0; (i) < (int)(a); (i)++)
//a -> b-1
#define reps(i, a, b) for (int i = (int)(a); (i) < (int)(b); (i)++)
//a-1 -> 0
#define rrep(i, a) for (int i = (int)a - 1; (i) >= 0; (i)--)
//a-1 -> b
#define rreps(i, a, b) for (int i = (int)(a)-1; (i) >= (int)(b); (i)--)
#define FOR(x, vec) for (auto& x : vec)
#define MP(a, b) make_pair((a), (b))
#define PB(a) push_back((a))
#define all(v) (v).begin(), (v).end()
//next_permutation(all(v))
#define PERM(v) next_permutation(all(v))
/*sort(all(v));
(v).erase(unique(all(v)), v.end())*/
#define UNIQUE(v) \
	sort(all(v)); \
	(v).erase(unique(all(v)), v.end())
#define CIN(type, x) \
	type x;          \
	cin >> x
#define YES(f) if ((f)) {cout << "YES" << endl;} else {cout << "NO" << endl;}
#define Yes(f) if ((f)) {cout << "Yes" << endl;} else {cout << "No" << endl;}
#define MINV(v) min_element(all(v))
#define MAXV(v) max_element(all(v))
#define MIN3(a, b, c) min(min(a, b), c)
#define MIN4(a, b, c, d) min(MIN3(a, b, c), d)
#define MIN5(a, b, c, d, e) min(MIN4(a, b, c, d), e)
#define MIN6(a, b, c, d, e, f) min(MIN5(a, b, c, d, e), f)
#define MAX3(a, b, c) max(max(a, b), c)
#define MAX4(a, b, c, d) max(MAX3(a, b, c), d)
#define MAX5(a, b, c, d, e) max(MAX4(a, b, c, d), e)
#define MAX6(a, b, c, d, e, f) max(MAX5(a, b, c, d, e), f)
//b is [a, c)
#define RANGE(a, b, c) ((a) <= (b) && (b) < (c))
//c is [a, e) && d is [b, f)
#define RANGE2D(a, b, c, d, e, f) (RANGE((a), (c), (e)) && RANGE((b), (d), (f)))
#define chmin(a, b) a = min(a, (b))
#define chmin3(a, b, c) a = MIN3(a, (b), (c))
#define chmin4(a, b, c, d) a = MIN4(a, (b), (c), (d))
#define chmin5(a, b, c, d, e) a = MIN5(a, (b), (c), (d), (e))
#define chmin6(a, b, c, d, e, f) a = MIN6(a, (b), (c), (d), (e), (f))
#define chmax(a, b) a = max(a, (b))
#define chmax3(a, b, c) a = MAX3(a, (b), (c))
#define chmax4(a, b, c, d) a = MAX4(a, (b), (c), (d))
#define chmax5(a, b, c, d, e) a = MAX5(a, (b), (c), (d), (e))
#define chmax6(a, b, c, d, e, f) a = MAX6(a, (b), (c), (d), (e), (f))
#define fcout cout << fixed << setprecision(12)
#define RS resize
#define CINV(v, N)             \
	do                         \
	{                          \
		v.RS(N);               \
		rep(i, N) cin >> v[i]; \
	} while (0);

#define RCINV(v, N)             \
	do                          \
	{                           \
		v.RS(N);                \
		rrep(i, N) cin >> v[i]; \
	} while (0);
#define MOD 1000000007

template <class T>
inline T GET()
{
	T x;
	cin >> x;
	return x;
}

void init();
void solve();

signed main()
{
	init();
	solve();
}

vector<vector<vector<double>>> dp;
int N;

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

void init()
{
	cin >> N;
	dp.resize(N + 1, vector<vector<double>>(N + 1, vector<double>(N + 1, -1)));
	dp[0][0][0] = 0;
	vector<int> v(3, 0);
	rep(i, N) {
		int a;
		cin >> a;
		v[a - 1]++;
	}
	fcout << dfs(v[0], v[1], v[2]) << endl;
}

void solve()
{
}


Measured memory_bytes: 216960