import java.util.Scanner;

public class taking_balls {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int N = sc.nextInt();
        int ones = 0, twos = 0, threes = 0;
        for (int i = 0; i < N; i++) {
            int x = sc.nextInt();
            if (x == 1) ones++;
            else if (x == 2) twos++;
            else if (x == 3) threes++;
        }

        double[][][] dp = 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++) {
                    dp[i][j][k] = 0.0;
                }
            }
        }

        for (int k = 0; k <= N; k++) {
            for (int j = 0; j <= N; j++) {
                for (int i = 0; i <= N; i++) {
                    if (i + j + k > N || i + j + k == 0) continue;
                    if (i > 0) dp[i][j][k] += i * dp[i - 1][j][k] / (i + j + k);
                    if (j > 0) dp[i][j][k] += j * dp[i + 1][j - 1][k] / (i + j + k);
                    if (k > 0) dp[i][j][k] += k * dp[i][j + 1][k - 1] / (i + j + k);
                    dp[i][j][k] += (double) N / (i + j + k);
                }
            }
        }

        System.out.printf("%.12f\n", dp[ones][twos][threes]);
    }
}