import java.io.*;
import java.util.*;


public class Main {

	private static Scanner sc;
	private static Printer pr;

	private static void solve() {
		int n = sc.nextInt();

		int[] cnt = new int[3];
		for (int i = 0; i < n; i++) {
			int a = sc.nextInt();
			cnt[a - 1]++;
		}

		pr.printf("%.10f\n", dp(cnt[0], cnt[1], cnt[2], n));
	}
	
	private static double[][][] dp;
	
	private static double dp(int cnt0, int cnt1, int cnt2, int n) {
		if (dp == null) {
			dp = new double[n + 1][n + 1][n + 1];
			for (int i = 0; i <= n; i++) {
				for (int j = 0; j <= n; j++) {
					Arrays.fill(dp[i][j], -1);
				}
			}
			dp[0][0][0] = 0;
		}

		if (dp[cnt0][cnt1][cnt2] >= 0) {
			return dp[cnt0][cnt1][cnt2];
		}
					
		double tmp = 1;
		if (cnt0 > 0) {
			tmp += dp(cnt0 - 1, cnt1, cnt2, n) * cnt0 / n;
		}
		if (cnt1 > 0) {
			tmp += dp(cnt0 + 1, cnt1 - 1, cnt2, n) * cnt1 / n;
		}
		if (cnt2 > 0) {
			tmp += dp(cnt0, cnt1 + 1, cnt2 - 1, n) * cnt2 / n;
		}
		tmp *= n;
		tmp /= (cnt0 + cnt1 + cnt2);

		dp[cnt0][cnt1][cnt2] = tmp;
		return tmp;
	}

	static class Tri implements Comparable<Tri> {
		int a;
		int b;
		int c;

		Tri(int a, int b, int c) {
			this.a = a;
			this.b = b;
			this.c = c;
		}

		@Override
		public int compareTo(Tri o) {
			if (a == o.a) {
				if (b == o.b) {
					return Integer.compare(c, o.c);
				}

				return Integer.compare(b, o.b);
			}

			return Integer.compare(a, o.a);
		}

		@Override
		public int hashCode() {
			return Integer.hashCode(a + b + c);
		}

		@Override
		public boolean equals(Object obj) {
			Tri o = (Tri) obj;
			return a == o.a && b == o.b && c == o.c;
		}

		@Override
		public String toString() {
			// [xxx, xxxx]
			StringBuilder stmp = new StringBuilder(64);
			stmp.append('[');
			stmp.append(a);
			stmp.append(',');
			stmp.append(' ');
			stmp.append(b);
			stmp.append(',');
			stmp.append(' ');
			stmp.append(c);
			stmp.append(']');

			return stmp.toString();
		}
	}

    // ---------------------------------------------------
	public static void main(String[] args) {
		sc = new Scanner(INPUT == null ? System.in : new ByteArrayInputStream(INPUT.getBytes()));
		pr = new Printer(System.out);

		solve();

//		pr.close();
		pr.flush();
//		sc.close();
	}

	static String INPUT = null;

	static class Scanner {
		BufferedReader br;

		Scanner (InputStream in) {
			br = new BufferedReader(new InputStreamReader(in));
		}

		private boolean isPrintable(int ch) {
			return ch >= '!' && ch <= '~';
		}

		private boolean isCRLF(int ch) {
			return ch == '\n' || ch == '\r' || ch == -1;
		}

		private int nextPrintable() {
			try {
				int ch;
				while (!isPrintable(ch = br.read())) {
					if (ch == -1) {
						throw new NoSuchElementException();
					}
				}

				return ch;
			} catch (IOException e) {
				throw new NoSuchElementException();
			}
		}

		String next() {
			try {
				int ch = nextPrintable();
				StringBuilder sb = new StringBuilder();
				do {
					sb.appendCodePoint(ch);
				} while (isPrintable(ch = br.read()));

				return sb.toString();
			} catch (IOException e) {
				throw new NoSuchElementException();
			}
		}

		int nextInt() {
			try {
				// parseInt from Integer.parseInt()
				boolean negative = false;
				int res = 0;
				int limit = -Integer.MAX_VALUE;
				int radix = 10;

				int fc = nextPrintable();
				if (fc < '0') {
					if (fc == '-') {
						negative = true;
						limit = Integer.MIN_VALUE;
					} else if (fc != '+') {
						throw new NumberFormatException();
					}
					fc = br.read();
				}
				int multmin = limit / radix;

				int ch = fc;
				do {
					int digit = ch - '0';
					if (digit < 0 || digit >= radix) {
						throw new NumberFormatException();
					}
					if (res < multmin) {
						throw new NumberFormatException();
					}
					res *= radix;
					if (res < limit + digit) {
						throw new NumberFormatException();
					}
					res -= digit;

				} while (isPrintable(ch = br.read()));

				return negative ? res : -res;
			} catch (IOException e) {
				throw new NoSuchElementException();
			}
		}

		long nextLong() {
			try {
				// parseLong from Long.parseLong()
				boolean negative = false;
				long res = 0;
				long limit = -Long.MAX_VALUE;
				int radix = 10;

				int fc = nextPrintable();
				if (fc < '0') {
					if (fc == '-') {
						negative = true;
						limit = Long.MIN_VALUE;
					} else if (fc != '+') {
						throw new NumberFormatException();
					}
					fc = br.read();
				}
				long multmin = limit / radix;

				int ch = fc;
				do {
					int digit = ch - '0';
					if (digit < 0 || digit >= radix) {
						throw new NumberFormatException();
					}
					if (res < multmin) {
						throw new NumberFormatException();
					}
					res *= radix;
					if (res < limit + digit) {
						throw new NumberFormatException();
					}
					res -= digit;

				} while (isPrintable(ch = br.read()));

				return negative ? res : -res;
			} catch (IOException e) {
				throw new NoSuchElementException();
			}
		}

		float nextFloat() {
			return Float.parseFloat(next());
		}

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

		String nextLine() {
			try {
				int ch;
				while (isCRLF(ch = br.read())) {
					if (ch == -1) {
						throw new NoSuchElementException();
					}
				}
				StringBuilder sb = new StringBuilder();
				do {
					sb.appendCodePoint(ch);
				} while (!isCRLF(ch = br.read()));

				return sb.toString();
			} catch (IOException e) {
				throw new NoSuchElementException();
			}
		}

		void close() {
			try {
				br.close();
			} catch (IOException e) {
//				throw new NoSuchElementException();
			}
		}
	}

	static class Printer extends PrintWriter {
		Printer(OutputStream out) {
			super(out);
		}
	}
}


Measured memory_bytes: 303828