import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.NoSuchElementException;

public class Main{
	static FastScanner sc = new FastScanner();
	static PrintWriter out = new PrintWriter(System.out);
	//static String S;
	//static String T;
	static int n;

	static double dp [][][];
	//static LinkedList <Integer> adj []; 
	
	public static void main(String[] args) {
		n = sc.nextInt();
		
		int c = 0;
		int one = 0; int two = 0; int three = 0;
		for (int i=0; i<n; i++) {
			c = sc.nextInt();
			if (c==1)
				one++;
			
			else if (c==2)
				two++;
			
			else if (c==3)
				three++;
		}
		
		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] = -1;
				}
			}
		}
		
		out.println(rec(one, two, three));
		
		out.flush();
	}
	
	static double rec (int i, int j, int k) {
		if (dp[i][j][k]>=0)
			return dp[i][j][k];
		
		if (i==0 && j==0 && k==0)
			return 0.0;
		
		double ans = 0.0;
		
		if (i>0)
			ans += rec (i-1, j, k) * i;
		
		if (j>0)
			ans += rec (i+1, j-1, k) * j;
		
		if (k>0)
			ans += rec (i, j+1, k-1) * k;
		
		ans += (double)n;
		ans *= (1.0/(i+j+k));
		
		return dp[i][j][k] = ans;
	}
	
	
		static class FastScanner {
		    private final InputStream in = System.in;
		    private final byte[] buffer = new byte[1024];
		    private int ptr = 0;
		    private int buflen = 0;

		    private boolean hasNextByte() {
		      if (ptr < buflen) {
		        return true;
		      } else {
		        ptr = 0;
		        try {
		          buflen = in.read(buffer);
		        } catch (IOException e) {
		          e.printStackTrace();
		        }
		        if (buflen <= 0) {
		          return false;
		        }
		      }
		      return true;
		    }

		    private int readByte() {
		      if (hasNextByte()) return buffer[ptr++];
		      else return -1;
		    }

		    private static boolean isPrintableChar(int c) {
		      return 33 <= c && c <= 126;
		    }

		    private void skipUnprintable() {
		      while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;
		    }

		    public boolean hasNext() {
		      skipUnprintable();
		      return hasNextByte();
		    }

		    public String next() {
		      if (!hasNext()) throw new NoSuchElementException();
		      StringBuilder sb = new StringBuilder();
		      int b = readByte();
		      while (isPrintableChar(b)) {
		        sb.appendCodePoint(b);
		        b = readByte();
		      }
		      return sb.toString();
		    }

		    public long nextLong() {
		      if (!hasNext()) throw new NoSuchElementException();
		      long n = 0;
		      boolean minus = false;
		      int b = readByte();
		      if (b == '-') {
		        minus = true;
		        b = readByte();
		      }
		      if (b < '0' || '9' < b) {
		        throw new NumberFormatException();
		      }
		      while (true) {
		        if ('0' <= b && b <= '9') {
		          n *= 10;
		          n += b - '0';
		        } else if (b == -1 || !isPrintableChar(b)) {
		          return minus ? -n : n;
		        } else {
		          throw new NumberFormatException();
		        }
		        b = readByte();
		      }
		    }

		    public int nextInt() {
		      return (int) nextLong();
		    }
		    
		    public double nextDouble(){
		    	return Double.parseDouble(next());
		    }

		    public int[] nextIntArray(int N, boolean oneBased) {
		      if (oneBased) {
		        int[] array = new int[N + 1];
		        for (int i = 1; i <= N; i++) {
		          array[i] = sc.nextInt();
		        }
		        return array;
		      } else {
		        int[] array = new int[N];
		        for (int i = 0; i < N; i++) {
		          array[i] = sc.nextInt();
		        }
		        return array;
		      }
		    }

		    public long[] nextLongArray(int N, boolean oneBased) {
		      if (oneBased) {
		        long[] array = new long[N + 1];
		        for (int i = 1; i <= N; i++) {
		          array[i] = sc.nextLong();
		        }
		        return array;
		      } else {
		        long[] array = new long[N];
		        for (int i = 0; i < N; i++) {
		          array[i] = sc.nextLong();
		        }
		        return array;
		      }
		    }
		  }

		}	 





Measured memory_bytes: 304316