#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <cstdio>
#include <set>
using namespace std;
#define rep(i,n) for(int i=0;i<(n);i++)
const int INF = 1e9;

int n, a[12];
double dp[102][102][102];

double solve(int c3, int c2, int c1){
	if(dp[c3][c2][c1] != -1) return dp[c3][c2][c1];

	//式変形後
	double ret = (double)n / (double)(c1 + c2 + c3);
	if(c3 > 0) ret += solve(c3 - 1, c2 + 1, c1) * (double)c3 / (double)(c1 + c2 + c3);
	if(c2 > 0) ret += solve(c3, c2 - 1, c1 + 1) * (double)c2 / (double)(c1 + c2 + c3);
	if(c1 > 0) ret += solve(c3, c2, c1 - 1) * (double)c1 / (double)(c1 + c2 + c3);
	return dp[c3][c2][c1] = ret;
}

int main(void){
	cin >> n;
	int cnt3 = 0, cnt2 = 0, cnt1 = 0;
	rep(i, n){
		cin >> a[i];
		if(a[i] == 0) cnt3++;
		else if(a[i] == 1) cnt2++;
		else if(a[i] == 2) cnt1++;
	}

	rep(i, 102)rep(j, 102)rep(k, 102) dp[i][j][k] = -1;
	dp[0][0][0] = 0.0;
	printf("%.12f\n", solve(cnt3, cnt2, cnt1));
	return 0;
}