import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.InputMismatchException;
import java.io.IOException;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        FastScanner in = new FastScanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        TaskJ solver = new TaskJ();
        solver.solve(1, in, out);
        out.close();
    }

    static class TaskJ {
        public void solve(int testNumber, FastScanner in, PrintWriter out) {
            int n = in.nextInt();
            double[][][] dp = new double[n + 1][n + 1][n + 1];

            int c[] = new int[4];
            for (int i = 0; i < n; i++)
                c[in.nextInt()]++;

            for (int c3 = 0; c3 <= n; c3++) {
                for (int c2 = 0; (c2 + c3) <= n; c2++) {
                    for (int c1 = (c2 | c3) == 0 ? 1 : 0; (c1 + c2 + c3) <= n; c1++) {
                        int s = c1 + c2 + c3;
                        dp[c1][c2][c3] = (double) n / s;
                        if (c3 > 0)
                            dp[c1][c2][c3] += dp[c1][c2 + 1][c3 - 1] * c3 / s;
                        if (c2 > 0)
                            dp[c1][c2][c3] += dp[c1 + 1][c2 - 1][c3] * c2 / s;
                        if (c1 > 0)
                            dp[c1][c2][c3] += dp[c1 - 1][c2][c3] * c1 / s;
                    }
                }
            }

            out.println(dp[c[1]][c[2]][c[3]]);
        }

    }

    static class FastScanner {
        private InputStream in;
        private byte[] buffer = new byte[1024];
        private int bufPointer;
        private int bufLength;

        public FastScanner(InputStream in) {
            this.in = in;
        }

        private int readByte() {
            if (bufPointer >= bufLength) {
                if (bufLength == -1)
                    throw new InputMismatchException();

                bufPointer = 0;
                try {
                    bufLength = in.read(buffer);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }

                if (bufLength <= 0)
                    return -1;
            }
            return buffer[bufPointer++];
        }

        private static boolean isSpaceChar(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public long nextLong() {
            long n = 0;

            int b = readByte();
            while (isSpaceChar(b))
                b = readByte();

            boolean minus = (b == '-');
            if (minus)
                b = readByte();

            while (b >= '0' && b <= '9') {
                n *= 10;
                n += b - '0';
                b = readByte();
            }

            if (!isSpaceChar(b))
                throw new NumberFormatException();

            return minus ? -n : n;
        }

        public int nextInt() {
            long n = nextLong();

            if (n < Integer.MIN_VALUE || n > Integer.MAX_VALUE)
                throw new NumberFormatException();

            return (int) n;
        }

    }
}



Measured memory_bytes: 303956