import java.util.*;

import java.io.*;
import java.math.BigInteger;

public class Main {
	FastScanner in;
	PrintWriter out;
	boolean systemIO = true;

	public long pow(long x, long p, long mod) {
		if (p <= 0) {
			return 1;
		}
		long th = pow(x, p / 2, mod);
		th *= th;
		th %= mod;
		if (p % 2 == 1) {
			th *= x;
		}
		return th;
	}

	public class Fraction implements Comparable<Fraction> {
		long x;
		long y;

		public Fraction(long x, long y) {
			this.x = x;
			this.y = y;
		}

		public void norm() {
			long gcd = gcd(x, y);
			x /= gcd;
			y /= gcd;
		}

		@Override
		public int compareTo(Fraction o) {
			if (x > o.x) {
				return 1;
			}
			if (x < o.x) {
				return -1;
			}
			if (y > o.y) {
				return 1;
			}
			if (y < o.y) {
				return -1;
			}
			return 0;
		}
	}

	public Fraction sum(Fraction a, Fraction b) {
		Fraction c = new Fraction(a.x * b.y + a.y * b.x, a.y * b.y);
		c.norm();
		return c;
	}

	public long gcd(long x, long y) {
		if (y == 0) {
			return x;
		}
		if (x == 0) {
			return y;
		}
		return gcd(y, x % y);
	}

	public class Pair implements Comparable<Pair> {
		long x;
		long y;

		public Pair(long x, long y) {
			this.x = x;
			this.y = y;
		}

		@Override
		public int compareTo(Pair o) {
			// if (x != o.x) {
			// return x - o.x;
			// }
			// return y - o.y;
			return 0;
		}
	}

	// public class Vertex {
	// Vertex father;
	// Vertex l;
	// Vertex r;
	// int sz;
	// long y;
	// long x;
	// int time;
	// int mintime;
	// int value;
	//
	// public Vertex(Vertex father, Vertex l, Vertex r, int sz, long y, long x, int
	// time, int mintime, int value) {
	// this.father = father;
	// this.l = l;
	// this.r = r;
	// this.sz = sz;
	// this.y = y;
	// this.x = x;
	// this.time = time;
	// this.mintime = mintime;
	// this.value = value;
	// }
	//
	// public Vertex(long x, int time, int value) {
	// this.father = null;
	// this.l = null;
	// this.r = null;
	// this.sz = 1;
	// this.y = random.nextLong();
	// this.x = x;
	// this.time = time;
	// this.mintime = time;
	// this.value = value;
	// }
	//
	// public void update() {
	// sz = 1;
	// mintime = time;
	// if (l != null) {
	// sz += l.sz;
	// mintime = Math.min(time, mintime);
	// }
	// if (r != null) {
	// sz += r.sz;
	// mintime = Math.min(time, mintime);
	// }
	// }
	// }
	//
	// public void add0(Vertex v, Vertex v1, Vertex father) {
	// if (v == null) {
	// v1.father = father;
	// father.r = v1;
	// father.update();
	// return;
	// }
	// if (v.y >= v1.y) {
	// add0(v.r, v1, v);
	// father.update();
	// return;
	// }
	// father.r = v1;
	// v1.father = father;
	// v.father = v1;
	// v1.l = v;
	// v1.update();
	// father.update();
	// }
	//
	// public Vertex pop(Vertex v, Vertex father, int h) {
	// if (v.l == null) {
	// if (h == 0) {
	// father.r = v.r;
	// } else {
	// father.l = v.r;
	// }
	// if (v.r != null) {
	// v.r.father = father;
	// v.r.update();
	// }
	// father.update();
	// v.father = null;
	// v.r = null;
	// v.update();
	// return v;
	// }
	// Vertex ans = pop(v.l, v, h + 1);
	// father.update();
	// return ans;
	// }
	//
	// public void cut(Vertex rl, Vertex rr, Vertex v, int mint) {
	//
	// }
	//
	// public int push(Vertex v, Vertex v1, Vertex father, boolean rson, int szl) {
	//
	// }
	//
	// Vertex root0;

	// public class State implements Comparable<State> {
	// int[] a;
	//
	// public State(int[] a) {
	// this.a = a;
	// }
	//
	// @Override
	// public int compareTo(State o) {
	// for (int i = 0; i < a.length; i++) {
	// if (a[i] != o.a[i]) {
	// return a[i] - o.a[i];
	// }
	// }
	// return 0;
	// }
	// }

	long mod = 1000000007;
	int n;

	public void solve() {
		n = in.nextInt();
		int[] a = new int[4];
		for (int i = 0; i < n; i++) {
			a[in.nextInt()]++;
		}
		long time = System.currentTimeMillis();
//		n = 300;
		double[][][] dp = new double[n + 1][n + 1][n + 1];
		System.err.println(System.currentTimeMillis() - time);
		int c = 0;
		for (int sum = 0; sum < dp.length; sum++) {
			for (int i = 0; i <= sum; i++) {
				for (int j = 0; i + j <= sum; j++) {
					c++;
					int[] th = { n - sum, sum - i - j, j, i };
					if (th[1] + th[2] + th[3] > 0) {
						dp[th[1]][th[2]][th[3]] += 1.0 * th[0] / (n - th[0]);
					}
					if (th[0] > 0) {
						dp[th[1] + 1][th[2]][th[3]] += (dp[th[1]][th[2]][th[3]] + 1) * (th[1] + 1)
								/ (th[1] + 1 + th[2] + th[3]);
					}
					if (th[1] > 0) {
						dp[th[1] - 1][th[2] + 1][th[3]] += (dp[th[1]][th[2]][th[3]] + 1) * (th[2] + 1)
								/ (th[1] + th[2] + th[3]);
					}
					if (th[2] > 0) {
						dp[th[1]][th[2] - 1][th[3] + 1] += (dp[th[1]][th[2]][th[3]] + 1) * (th[3] + 1)
								/ (th[1] + th[2] + th[3]);
					}
				}
			}
		}
		out.print(dp[a[1]][a[2]][a[3]]);
		System.err.println(c);
		System.err.println(System.currentTimeMillis() - time);
	}

	// public long convert(int[] th) {
	// long ans = 0;
	// for (int i = 0; i < th.length; i++) {
	// ans *= n;
	// ans += th[i];
	// }
	// return ans;
	// }
	//
	// public int[] convert(long x) {
	// int[] th = new int[4];
	// th[3] = (int) (x % n);
	// x /= n;
	// th[2] = (int) (x % n);
	// x /= n;
	// th[1] = (int) (x % n);
	// x /= n;
	// th[0] = (int) (x % n);
	// return th;
	// }
	//
	// long mod = 1000000007;
	// int n;
	//
	// public void solve() {
	// n = in.nextInt();
	// int[] a = new int[4];
	// for (int i = 0; i < n; i++) {
	// a[in.nextInt()]++;
	// }
	// long time = System.currentTimeMillis();
	//// n = 300;
	// n++;
	// double[][][] dp = new double[n][n][n];
	// System.err.println(System.currentTimeMillis() - time);
	// TreeSet<Long> set = new TreeSet<>();
	// int[] st = {n - 1, 0, 0, 0};
	// set.add(convert(st));
	// int c = 0;
	// while (!set.isEmpty()) {
	// c++;
	// int[] th = convert(set.pollLast());
	// if (th[1] + th[2] + th[3] > 0) {
	// dp[th[1]][th[2]][th[3]] += 1.0 * th[0] / (n - th[0]);
	// }
	// if (th[0] > 0) {
	// dp[th[1] + 1][th[2]][th[3]] += (dp[th[1]][th[2]][th[3]] + 1) * (th[1] + 1) /
	// (th[1] + 1 + th[2] + th[3]);
	// int[] s = {th[0] - 1, th[1] + 1, th[2], th[3]};
	// set.add(convert(s));
	// }
	// if (th[1] > 0) {
	// dp[th[1] - 1][th[2] + 1][th[3]] += (dp[th[1]][th[2]][th[3]] + 1) * (th[2] +
	// 1) / (th[1] + th[2] + th[3]);
	// int[] s = {th[0], th[1] - 1, th[2] + 1, th[3]};
	// set.add(convert(s));
	// }
	// if (th[2] > 0) {
	// dp[th[1]][th[2] - 1][th[3] + 1] += (dp[th[1]][th[2]][th[3]] + 1) * (th[3] +
	// 1) / (th[1] + th[2] + th[3]);
	// int[] s = {th[0], th[1], th[2] - 1, th[3] + 1};
	// set.add(convert(s));
	// }
	// }
	// out.print(dp[a[1]][a[2]][a[3]]);
	// System.err.println(c);
	// System.err.println(System.currentTimeMillis() - time);
	// }

	public void solve1() {
		String s = in.next();
		String t = in.next();
		int[][] ans = new int[s.length() + 1][t.length() + 1];
		for (int i = 0; i < ans.length - 1; i++) {
			for (int j = 0; j < ans[0].length - 1; j++) {
				if (s.charAt(i) == t.charAt(j)) {
					ans[i + 1][j + 1] = ans[i][j] + 1;
				}
			}
		}
		String answer = "";
		for (int i = 0; i < ans.length; i++) {
			for (int j = 0; j < ans[0].length; j++) {
				if (ans[i][j] > answer.length()) {
					answer = s.substring(i - ans[i][j], i);
				}
			}
		}
		out.print(answer);
	}

	public void run() {
		try {
			if (systemIO) {
				in = new FastScanner(System.in);
				out = new PrintWriter(System.out);
			} else {
				in = new FastScanner(new File("input.txt"));
				out = new PrintWriter(new File("output.txt"));
			}
			solve();

			out.close();
		} catch (IOException e) {
			e.printStackTrace();
		}
	}

	class FastScanner {
		BufferedReader br;
		StringTokenizer st;

		FastScanner(File f) {
			try {
				br = new BufferedReader(new FileReader(f));
			} catch (FileNotFoundException e) {
				e.printStackTrace();
			}
		}

		FastScanner(InputStream f) {
			br = new BufferedReader(new InputStreamReader(f));
		}

		String nextLine() {
			try {
				return br.readLine();
			} catch (IOException e) {
				return null;
			}
		}

		String next() {
			while (st == null || !st.hasMoreTokens()) {
				try {
					st = new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
			return st.nextToken();
		}

		int nextInt() {
			return Integer.parseInt(next());
		}

		long nextLong() {
			return Long.parseLong(next());
		}

		double nextDouble() {
			return Double.parseDouble(next());
		}

	}

	// AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
	public static void main(String[] arg) {
		new Main().run();
	}
}

Measured memory_bytes: 305364