import java.util.*;

public class Main {
    private static double solve(int N, int[] a) {
        long state = 0;
        long[] p = new long[] {1, 300, 300*300, 300*300*300, 300*300*300*300 };
        for (int i = 0; i < N; i++) state += p[a[i]];
        return solve(N, state, new HashMap<Long, Double>());
    }

    private static double solve(int N, long state, Map<Long, Double> memo) {
        if (state == 0) return 1;
        if (memo.containsKey(state)) return memo.get(state);

        long[] count = new long[4];
        long temp = state;
        for (int i = 0; i < 3; i++)  {
             count[i] = temp % 300;
             temp /= 300;
        }

        double e = 0;
        long p = 1;
        for (int i = 0; i < 3; i++, p *= 300) {
            if (count[i] == 0) continue;

            e += ((double)N/count[i])*solve(N, state - p, memo);
        }
        memo.put(state, e);
        return e;
    }

    public static double run(Scanner scanner) {
        int N = scanner.nextInt();
        int[] a = new int[N];
        for (int i=0; i < N; i++) a[i] = scanner.nextInt();

        return solve(N, a);
    }

    public static void main(String[] args) {
        try (Scanner scanner = new Scanner(System.in)) {
            System.out.println(run(scanner));
        }
        Tests.run();
    }
}

class Tests {
    public static void run() {
        testCase("3\n" +
                "1 1 1", 5.5);
        testCase("1\n" +
                "3", 3);
        testCase("2\n" +
                "1 2", 4.5);
        testCase("10\n" +
                "1 3 2 3 3 2 3 2 1 3", 54.48064457488221);

        System.out.println("DONE");
    }

    private static void testCase(String input, double expected) {
        try (Scanner scanner = new Scanner(input)) {
            double result = Main.run(scanner);
            if (Math.abs(result-expected) > 1e-9) System.out.println("TEST FAILED: was " + result + " expected " + expected + " on input\n" + input);
        }
    }
}