#define _CRT_SECURE_NO_WARNINGS
#include <string>
#include <vector>
#include <algorithm>
#include <numeric>
#include <set>
#include <map>
#include <queue>
#include <iostream>
#include <sstream>
#include <cstdio>
#include <cmath>
#include <ctime>
#include <cstring>
#include <cctype>
#include <cassert>
#include <limits>
#include <functional>
#define rep(i,n) for(int (i)=0;(i)<(int)(n);++(i))
#define rer(i,l,u) for(int (i)=(int)(l);(i)<=(int)(u);++(i))
#define reu(i,l,u) for(int (i)=(int)(l);(i)<(int)(u);++(i))
#if defined(_MSC_VER) || __cplusplus > 199711L
#define aut(r,v) auto r = (v)
#else
#define aut(r,v) typeof(v) r = (v)
#endif
#define each(it,o) for(aut(it, (o).begin()); it != (o).end(); ++ it)
#define all(o) (o).begin(), (o).end()
#define pb(x) push_back(x)
#define mp(x,y) make_pair((x),(y))
#define mset(m,v) memset(m,v,sizeof(m))
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3fLL
using namespace std;
typedef vector<int> vi; typedef pair<int,int> pii; typedef vector<pair<int,int> > vpii;
typedef long long ll; typedef vector<long long> vl; typedef pair<long long,long long> pll; typedef vector<pair<long long,long long> > vpll;
typedef vector<string> vs; typedef long double ld;
template<typename T, typename U> inline void amin(T &x, U y) { if(y < x) x = y; }
template<typename T, typename U> inline void amax(T &x, U y) { if(x < y) x = y; }

int N;
double memo[101][101][101];

double rec(int n0, int n1, int n2) {
	if(n0 < 0 || n1 < 0 || n2 < 0) return 0;
	double &r = memo[n0][n1][n2];
	if(r == r) return r;
	int sum = n0 + n1 + n2;
	if(sum == 0) return r = 0;
	double p0 = n0 * 1. / sum;
	double p1 = n1 * 1. / sum;
	double p2 = n2 * 1. / sum;
	double q = sum * 1. / N;
	r = 1 / q + p0 * rec(n0-1, n1+1, n2) + p1 * rec(n0, n1-1, n2+1) + p2 * rec(n0, n1, n2-1);
	return r;
}

int main() {
	scanf("%d", &N);
	vector<int> A(N);
	rep(i, N) scanf("%d", &A[i]);
	int cnt0 = 0, cnt1 = 0, cnt2 = 0;
	rep(i, N) {
		if(A[i] == 0)
			++ cnt0;
		else if(A[i] == 1)
			++ cnt1;
		else if(A[i] == 2)
			++ cnt2;
	}
	mset(memo, -1);
	double ans = rec(cnt0, cnt1, cnt2);
	printf("%.10f\n", ans);
	return 0;
}