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;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        JSushi solver = new JSushi();
        solver.solve(1, in, out);
        out.close();
    }

    static class JSushi {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int N = in.nextInt();
            int[] c = new int[4];
            double[][][] prob = new double[N + 1][N + 1][N + 1];
            double[][][] eval = new double[N + 1][N + 1][N + 1];
            for (int i = 0; i < N; i++) {
                c[in.nextInt()]++;
            }

            prob[c[1]][c[2]][c[3]] = 1;

            for (int k = N; k >= 0; k--) {
                for (int j = N; j >= 0; j--) {
                    for (int i = N; i >= 0; i--) {
                        if ((i == 0 && j == 0 && k == 0) || i + j + k > N) {
                            continue;
                        }

                        double pw = (double) (N - i - j - k) / N;
                        eval[i][j][k] += (1 / (1 - pw)) * prob[i][j][k];

                        if (i != 0) {
                            prob[i - 1][j][k] += prob[i][j][k] * ((double) i / (i + j + k));
                            eval[i - 1][j][k] += eval[i][j][k] * ((double) i / (i + j + k));
                        }
                        if (j != 0) {
                            prob[i + 1][j - 1][k] += prob[i][j][k] * ((double) j / (i + j + k));
                            eval[i + 1][j - 1][k] += eval[i][j][k] * ((double) j / (i + j + k));
                        }
                        if (k != 0) {
                            prob[i][j + 1][k - 1] += prob[i][j][k] * ((double) k / (i + j + k));
                            eval[i][j + 1][k - 1] += eval[i][j][k] * ((double) k / (i + j + k));
                        }
                    }
                }
            }
            out.print(eval[0][0][0]);


        }

    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        public int read() {
            if (numChars == -1)
                throw new InputMismatchException();
            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (numChars <= 0)
                    return -1;
            }
            return buf[curChar++];
        }

        public int nextInt() {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            int res = 0;
            do {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

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

    }
}



Measured memory_bytes: 596436