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.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.*;
import java.io.*;
public class Main {
InputStream is;
	PrintWriter out;
	String INPUT = ""; 
//class  Declaration

static class pair implements Comparable<pair>{
	int x;
	int y;
	
	
	pair (int i,int j)
	{ x=i; y=j;
		
	}
	public int compareTo(pair p){
		if(this.x!=p.x) { return this.x-p.x;}
		else { return this.y-p.y;}
	}
	public int hashCode() { return (x+" "+y).hashCode();}
	public String toString(){ return x+" "+y;} 
	public boolean equals(Object o){ 
		pair x = (pair) o ;
		return (x.x==this.x&&x.y==this.y);}
}

long mod  = (long)1e9 + 7 ;
double [][][] dp = new double[301][301][301];
int n ;
int[] arr  =new int[301];
boolean[][][] solved = new boolean[301][301][301]; 
void solve() throws Exception{
 n= ni();

for(int i=1;i<=n;++i) arr[i]=ni();

solved[0][0][0] = true ; 

int one = 0 , two = 0,three = 0 ;
for(int i=1;i<=n;++i){
	if(arr[i] == 1 ) one++;
	if(arr[i]==2 ) two++ ;
	if(arr[i]==3) three++;
}
pn(recur(one,two,three));





}
double recur(int i,int j,int k){
	//pn("trying "+i+" "+j+" "+k);
	if(solved[i][j][k]) return dp[i][j][k]; 
	double mul = (n*1.0)/(i+j+k) ; 
	double sum  = 0.0  ;
	// if(i!=0){
	// 	sum += ((i*1.0)/n)*(recur(i-1,j,k)+1.0);
	// }
	// if(j!=0){
	// 	sum += ((j*1.0)/n)*(recur(i+1,j-1,k)+1.0);
	// }
	// if(k!=0){
	// 	sum += ((k*1.0)/n)*(recur(i,j+1,k-1)+1.0);
	// }

	if(i!=0){
		sum += ((i*1.0)/(i+j+k))*(recur(i-1,j,k)+ mul);
	}
	if(j!=0){
		sum += (j*1.0)/(i+j+k)*(recur(i+1,j-1,k)+mul);
	}
	if(k!=0){
		sum += ((k*1.0)/(i+j+k))*(recur(i,j+1,k-1)+mul);
	}

	

	//sum = mul*sum ; 

	//pn("got : "+ sum);
	dp[i][j][k] =sum ;


	solved[i][j][k] = true ;
	return dp[i][j][k];

	
}

long pow(long a,long b){
	long result = 1;
	while(b>0){
	if(b%2==1) result = (result * a) % mod;
		b/=2;
		a=(a*a)%mod;
	}
	return result;
}

void print(Object o){
System.out.println(o);
System.out.flush();
}

int gcd(int a, int b) 
{ 
if (b == 0) 
return a; 
return gcd(b, a % b);  
}
void run() throws Exception{
		is = INPUT.isEmpty() ? System.in : new ByteArrayInputStream(INPUT.getBytes());
out = new PrintWriter(System.out);
		
		long s = System.currentTimeMillis();
		solve();
		out.flush();
		if(!INPUT.isEmpty())tr(System.currentTimeMillis()-s+"ms");
}
	
	
public static void main(String[] args) throws Exception { new Main().run(); }
	
//output methods
private void pn(Object o)
{
	out.println(o);
}
private void p(Object o)
{
	out.print(o);
}



//input methods
		private byte[] inbuf = new byte[1024];
	public int lenbuf = 0, ptrbuf = 0;
	

	private int readByte()
	{
		if(lenbuf == -1)throw new InputMismatchException();
		if(ptrbuf >= lenbuf){
			ptrbuf = 0;
			try { lenbuf = is.read(inbuf); } catch (IOException e) { throw new InputMismatchException(); }
			if(lenbuf <= 0)return -1;
		}
		return inbuf[ptrbuf++];
	}
	
	
	
	
	private boolean isSpaceChar(int c) { return !(c >= 33 && c <= 126); }
	private int skip() { int b; while((b = readByte()) != -1 && isSpaceChar(b)); return b; }
	
	private double nd() { return Double.parseDouble(ns()); }
	private char nc() { return (char)skip(); }
	
	private String ns()
	{
		int b = skip();
		StringBuilder sb = new StringBuilder();
		while(!(isSpaceChar(b))){ // when nextLine, (isSpaceChar(b) && b != ' ')
			sb.appendCodePoint(b);
			b = readByte();
		}
		return sb.toString();
	}
	
	private char[] ns(int n)
	{
		char[] buf = new char[n];
		int b = skip(), p = 0;
		while(p < n && !(isSpaceChar(b))){
			buf[p++] = (char)b;
			b = readByte();
		}
		return n == p ? buf : Arrays.copyOf(buf, p);
	}
		
	private char[][] nm(int n, int m)
	{
		char[][] map = new char[n][];
		for(int i = 0;i < n;i++)map[i] = ns(m);
		return map;
	}
	
	private int[] na(int n)
	{
		int[] a = new int[n];
		for(int i = 0;i < n;i++)a[i] = ni();
		return a;
	}
	
	private int ni()
	{
		int num = 0, b;
		boolean minus = false;
		while((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'));
		if(b == '-'){
			minus = true;
			b = readByte();
		}
		
		while(true){
			if(b >= '0' && b <= '9'){
				num = num * 10 + (b - '0');
			}else{
				return minus ? -num : num;
			}
			b = readByte();
		}
	}
	
	private long nl()
	{
		long num = 0;
		int b;
		boolean minus = false;
		while((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'));
		if(b == '-'){
			minus = true;
			b = readByte();
		}
		
		while(true){
			if(b >= '0' && b <= '9'){
				num = num * 10 + (b - '0');
			}else{
				return minus ? -num : num;
			}
			b = readByte();
		}
	}
	
	private void tr(Object... o) { if(INPUT.length() > 0)System.out.println(Arrays.deepToString(o)); }
	void watch(Object ... a) throws Exception{
		int i=1;
		print("watch starts :");
		for(Object o : a ) {
			//print(o);
			boolean notfound = true;
			if(o.getClass().isArray()){
				
				String type = o.getClass().getName().toString();
				//print("type is "+type);
				switch (type) {
					case "[I":{
						int[] test = (int[])o ;
						print(i+" "+Arrays.toString(test));
						break;
					}
					case "[[I":{
						int[][] obj = (int[][])o;
						
						print(i+" "+Arrays.deepToString(obj));
						break;
					}
					case "[J" : {
						
						long[] obj  = (long[])o ;
						print(i+" "+Arrays.toString(obj));
						break;
					}
					case "[[J": {
						
						long[][] obj = (long[][])o;
						print(i+" "+Arrays.deepToString(obj));
						break;
					}
					case "[D" :{
						
						double[] obj= (double[])o;
						print(i+" "+Arrays.toString(obj));
						break;
					}
					case "[[D" :{
						
						double[][] obj = (double[][])o;
						print(i+" "+Arrays.deepToString(obj));
						break;
					}
					case "[Ljava.lang.String": {
						
						String[] obj = (String[])o ;
						print(i+" "+Arrays.toString(obj));
						break;
					}
					case "[[Ljava.lang.String": {
						
						String[][] obj = (String[][])o ;
						print(i+" "+Arrays.deepToString(obj));
						break ; 
					}
					case "[C" :{
						char[] obj = (char[])o ;
						print(i+" "+Arrays.toString(obj));
						break;
					}
					case "[[C" :{
						
						char[][] obj = (char[][])o;
						print(i+" "+Arrays.deepToString(obj));
						break;
					}

						
				
					default:{
						print(i+" type not identified");
						break;
					}
				}
				notfound = false;
				
			}
			if(o.getClass() == ArrayList.class){
				print(i+" al: "+o);
				notfound = false;
			}
			if(o.getClass() == HashSet.class){
				print(i+" hs: "+o);
				notfound = false;
			}
			if(o.getClass() == TreeSet.class){
				print(i+" ts: "+o);
				notfound = false;
			}
			if(o.getClass() == TreeMap.class){
				print(i+" tm: "+o);
				notfound = false;
			}
			if(o.getClass() == HashMap.class){
				print(i+" hm: "+o);
				notfound = false;
			}
			if(o.getClass() == LinkedList.class){
				print(i+" ll: "+o);
				notfound = false;
			}
			if(o.getClass() == PriorityQueue.class){
				print(i+" pq : "+o);
				notfound = false;
			}
			if(o.getClass() == pair.class){
				print(i+" pq : "+o);
				notfound = false;
			}
			
			if(notfound){
				print(i+" unknown: "+o);
			}
			i++;
		}
		print("watch ends ");
	}

}