#ifdef LOCAL111
#else
	#define NDEBUG
#endif
#include <bits/stdc++.h>
const int INF = 1e9;
using namespace std;
template<typename T, typename U> ostream& operator<< (ostream& os, const pair<T,U>& p) { cout << '(' << p.first << ' ' << p.second << ')'; return os; }

#define endl '\n'
#define ALL(a)  (a).begin(),(a).end()
#define SZ(a) int((a).size())
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n)  FOR(i,0,n)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)
#define RBP(i,a) for(auto& i : a)
#ifdef LOCAL111
	#define DEBUG(x) cout<<#x<<": "<<(x)<<endl
	template<typename T> void dpite(T a, T b){ for(T ite = a; ite != b; ite++) cout << (ite == a ? "" : " ") << *ite; cout << endl;}
#else
	#define DEBUG(x) true
	template<typename T> void dpite(T a, T b){ return; }
#endif
#define F first
#define S second
#define SNP string::npos
#define WRC(hoge) cout << "Case #" << (hoge)+1 << ": "
#define rangej(a,b,c) ((a) <= (c) and (c) < (b))
#define rrangej(b,c) rangej(0,b,c)
template<typename T> void pite(T a, T b){ for(T ite = a; ite != b; ite++) cout << (ite == a ? "" : " ") << *ite; cout << endl;}
template<typename T> bool chmax(T& a, T b){if(a < b){a = b; return true;} return false;}
template<typename T> bool chmin(T& a, T b){if(a > b){a = b; return true;} return false;}

typedef long long int LL;
typedef unsigned long long ULL;
typedef pair<int,int> P;
typedef pair<LL,LL> LP;

void ios_init(){
	cout.setf(ios::fixed);
	cout.precision(12);
#ifdef LOCAL111
	return;
#endif
	ios::sync_with_stdio(false); cin.tie(0);	
}

int main()
{
	ios_init();
	int n;
	while(cin >> n){
		using vd = vector<double>;
		vector<int> a(n);
		REP(i,n) cin >> a[i];
		vector<vector<vd>> dp(n+1,vector<vd>(n+1,vd(n+1,-1)));
		vector<int> cnt(3,0);
		REP(i,n){
			if(a[i] < 3){
				cnt[2-a[i]]++;
			}
		}
		dpite(ALL(cnt));
		dp[0][0][0] = 0;
		function<double(int,int,int)> rec = [&](int p, int q, int r) -> double{
			// DEBUG(p); DEBUG(q); DEBUG(r);
			if(dp[p][q][r] != -1) return dp[p][q][r];
			double res = 0;
			int sum = p+q+r;
			int ho = n-sum;
			double d = double(ho)/n;
			if(p != 0){
				double a = (double)p/n;
				res += a/((1-d)*(1-d))+(a)*rec(p-1,q,r)/(1-d);
			}
			if(q != 0){
				double a = (double)q/n;
				res += a/((1-d)*(1-d))+(a)*rec(p+1,q-1,r)/(1-d);
			}
			if(r != 0){
				double a = (double)r/n;
				res += a/((1-d)*(1-d))+(a)*rec(p,q+1,r-1)/(1-d);
			}
			DEBUG(d); DEBUG((double)p/n); DEBUG(p); DEBUG(q); DEBUG(r);
			DEBUG(res); DEBUG(endl);
			return dp[p][q][r] = res;
		};
		cout << rec(cnt[0],cnt[1],cnt[2]) << endl;
	}
	return 0;
}