There are N dishes, numbered 1, 2, \ldots, N. Initially, for each i (1 \leq i \leq N), Dish i has a_i (1 \leq a_i \leq 3) pieces of sushi on it.

Taro will perform the following operation repeatedly until all the pieces of sushi are eaten:

* Roll a die that shows the numbers 1, 2, \ldots, N with equal probabilities, and let i be the outcome. If there are some pieces of sushi on Dish i, eat one of them; if there is none, do nothing.



Find the expected number of times the operation is performed before all the pieces of sushi are eaten.

Constraints

* All values in input are integers.
* 1 \leq N \leq 300
* 1 \leq a_i \leq 3

Input

Input is given from Standard Input in the following format:


N
a_1 a_2 \ldots a_N


Output

Print the expected number of times the operation is performed before all the pieces of sushi are eaten. The output is considered correct when the relative difference is not greater than 10^{-9}.

Examples

Input

3
1 1 1


Output

5.5


Input

1
3


Output

3


Input

2
1 2


Output

4.5


Input

10
1 3 2 3 3 2 3 2 1 3


Output

54.48064457488221
import java.io.*;
import java.math.BigDecimal;
import java.util.Arrays;
import java.util.StringJoiner;
import java.util.StringTokenizer;

public class Main {

    static int N;
    static int[] A;

    public static void main(String[] args) {
        FastScanner sc = new FastScanner(System.in);
        N = sc.nextInt();
        A = sc.nextIntArray(N);

        System.out.println(BigDecimal.valueOf(solve()).toPlainString());
    }

    static double solve() {
        int[] r = new int[3];
        for (int i = 0; i < N; i++) {
            r[A[i]-1]++;
        }

        MEMO = new double[N+1][N+1][N+1];
        for (int i = 0; i <= N; i++) {
            for (int j = 0; j <= N; j++) {
                for (int k = 0; k <= N; k++) {
                    MEMO[i][j][k] = -1;
                }
            }
        }

        return dfs(r[0], r[1], r[2]);
    }

    static double[][][] MEMO;

    static double dfs(int r1, int r2, int r3) {
        if( MEMO[r1][r2][r3] != -1 ) {
            return MEMO[r1][r2][r3];
        }

        // debug(r1, r2, r3, t);
        int r123 = r1 + r2 + r3;
        int r0 = N - r123;
        if( r0 == N ) return 0;

        double t = (double)N / r123;

        double ret = 0.0;
        if( r1 > 0 ) {
            double p1 = (double)r1 / r123;
            ret += (t + dfs(r1-1, r2, r3)) * p1;
        }
        if( r2 > 0 ) {
            double p2 = (double)r2 / r123;
            ret += (t + dfs(r1+1, r2-1, r3)) * p2;
        }
        if( r3 > 0 ) {
            double p3 = (double)r3 / r123;
            ret += (t + dfs(r1, r2+1, r3-1)) * p3;
        }
        MEMO[r1][r2][r3] = ret;
        return ret;
    }

    @SuppressWarnings("unused")
    static class FastScanner {
        private BufferedReader reader;
        private StringTokenizer tokenizer;

        FastScanner(InputStream in) {
            reader = new BufferedReader(new InputStreamReader(in));
            tokenizer = null;
        }

        String next() {
            if (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        String nextLine() {
            if (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    return reader.readLine();
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken("\n");
        }

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

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

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

        int[] nextIntArray(int n) {
            int[] a = new int[n];
            for (int i = 0; i < n; i++) a[i] = nextInt();
            return a;
        }

        int[] nextIntArray(int n, int delta) {
            int[] a = new int[n];
            for (int i = 0; i < n; i++) a[i] = nextInt() + delta;
            return a;
        }

        long[] nextLongArray(int n) {
            long[] a = new long[n];
            for (int i = 0; i < n; i++) a[i] = nextLong();
            return a;
        }
    }

    static void writeLines(int[] as) {
        PrintWriter pw = new PrintWriter(System.out);
        for (int a : as) pw.println(a);
        pw.flush();
    }

    static void writeLines(long[] as) {
        PrintWriter pw = new PrintWriter(System.out);
        for (long a : as) pw.println(a);
        pw.flush();
    }

    static void writeSingleLine(int[] as) {
        PrintWriter pw = new PrintWriter(System.out);
        for (int i = 0; i < as.length; i++) {
            if (i != 0) pw.print(" ");
            pw.print(i);
        }
        pw.println();
        pw.flush();
    }

    static int max(int... as) {
        int max = Integer.MIN_VALUE;
        for (int a : as) max = Math.max(a, max);
        return max;
    }

    static int min(int... as) {
        int min = Integer.MAX_VALUE;
        for (int a : as) min = Math.min(a, min);
        return min;
    }

    static void debug(Object... args) {
        StringJoiner j = new StringJoiner(" ");
        for (Object arg : args) {
            if (arg instanceof int[]) j.add(Arrays.toString((int[]) arg));
            else if (arg instanceof long[]) j.add(Arrays.toString((long[]) arg));
            else if (arg instanceof double[]) j.add(Arrays.toString((double[]) arg));
            else if (arg instanceof Object[]) j.add(Arrays.toString((Object[]) arg));
            else j.add(arg == null ? "null" : arg.toString());
        }
        System.err.println(j.toString());
    }

    static void printSingleLine(int[] array) {
        for (int i = 0; i < array.length; i++) {
            if (i != 0) System.out.print(" ");
            System.out.print(array[i]);
        }
        System.out.println();
    }

    static int lowerBound(int[] array, int value) {
        int low = 0, high = array.length, mid;
        while (low < high) {
            mid = ((high - low) >>> 1) + low;
            if (array[mid] < value) low = mid + 1;
            else high = mid;
        }
        return low;
    }

    static int upperBound(int[] array, int value) {
        int low = 0, high = array.length, mid;
        while (low < high) {
            mid = ((high - low) >>> 1) + low;
            if (array[mid] <= value) low = mid + 1;
            else high = mid;
        }
        return low;
    }
}
