#include<bits/stdc++.h>
#define maxn 320
using namespace std;
typedef long double LD;
LD f[maxn][maxn][maxn];
int c[3];
int main(){
	int N;
	cin >> N;
	for(int i = 0; i < N; i += 1){
		int a;
		cin >> a;
		c[a - 1] += 1;
	}
	for(int k = 0; k <= N; k += 1)
		for(int j = 0; j + k <= N; j += 1)
			for(int i = 0; i + j + k <= N; i += 1) if(i + j + k){
				f[i][j][k] = 1;
				if(i) f[i][j][k] += f[i - 1][j][k] * i / N;
				if(j) f[i][j][k] += f[i + 1][j - 1][k] * j / N;
				if(k) f[i][j][k] += f[i][j + 1][k - 1] * k / N;
				f[i][j][k] = f[i][j][k] / (i + j + k) * N;
			}
	cout << setprecision(15) << f[c[0]][c[1]][c[2]];
} 