package main

import (
	"bufio"
	"fmt"
	"io"
	"log"
	"os"
	"strconv"
)

var pri = fmt.Println
var scan = fmt.Fscanln
var scanone = fmt.Fscan

func main() {
	if os.Getenv("DEBUG") != "" {
		fname := "input.txt"
		f, err := os.Open(fname)
		if err != nil {
			log.Fatal(err)
		}
		r := bufio.NewReader(f)
		for {
			solve(r)
			if r.Buffered() == 0 {
				break
			}
		}
	} else {
		f := os.Stdin
		r := bufio.NewReader(f)
		solve(r)
	}
}

func min(a ...int64) int64 {
	res := a[0]
	for i := 1; i < len(a); i++ {
		if res > a[i] {
			res = a[i]
		}
	}
	return res
}

func max(a ...int64) int64 {
	res := a[0]
	for i := 1; i < len(a); i++ {
		if res < a[i] {
			res = a[i]
		}
	}
	return res
}

func mustint(a string) int64 {
	i, _ := strconv.Atoi(a)
	return int64(i)
}

type P struct {
	x, y int64
}

func (p *P) add(q *P) *P {
	return &P{p.x + q.x, p.y + q.y}
}

const huge = 1e10

func solve(r io.Reader) {
	var n int
	scan(r, &n)
	var x0, y0, z0 int
	for i := 0; i < n; i++ {
		var a int
		scanone(r, &a)
		switch a {
		case 0:
			x0++
		case 1:
			y0++
		case 2:
			z0++
		}
	}
	const N = 101
	var dp [N][N][N]float64
	for i := 0; i < N; i++ {
		for j := 0; j < N; j++ {
			for k := 0; k < N; k++ {
				dp[i][j][k] = -1
			}
		}
	}
	dp[0][0][0] = 0
	var f func(x, y, z int) float64
	f = func(x, y, z int) float64 {
		if dp[x][y][z] != -1 {
			return dp[x][y][z]
		}
		res := 0.0
		if x-1 >= 0 {
			res += float64(x) * f(x-1, y+1, z)
		}
		if y-1 >= 0 {
			res += float64(y) * f(x, y-1, z+1)
		}
		if z-1 >= 0 {
			res += float64(z) * f(x, y, z-1)
		}
		res += float64(n)
		res /= float64(x + y + z)
		dp[x][y][z] = res
		return res
	}
	pri(f(x0, y0, z0))
}