package no108;

import java.util.Scanner;

public class Main {
	static int n;
	static int[] a;
	static double[][][] dp;
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		n = sc.nextInt();
		a = new int[3];
		for(int i=0;i<n;i++) {
			int b = sc.nextInt();
			if (b < 3) {
				a[b]++;
			}
		}
		dp = new double[n+1][n+1][n+1];
		System.out.println(dfs(a[0],a[1],a[2]));
	}
	static double dfs(int n0,int n1,int n2) {
		if (n0 == 0 && n1 == 0 && n2 == 0) {
			return 0;
		}
		if (dp[n0][n1][n2] != 0) {
			return dp[n0][n1][n2];
		}
		double ret = 1;
		if (n0 > 0) {
			ret += dfs(n0-1,n1+1,n2) * ((double) n0/n);
		}
		if (n1 > 0) {
			ret += dfs(n0,n1-1,n2+1) * ((double) n1/n);
		}
		if (n2 > 0) {
			ret += dfs(n0,n1,n2-1) * ((double) n2/n);
		}
		return dp[n0][n1][n2] = ret / ((double) (n0 + n1 + n2) / n);
	}

}