#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i = 0; i < (n); ++i)
#define all(x) (x).begin(),(x).end()
#define ln '\n'
constexpr long long MOD = 1000000007LL;
//constexpr long long MOD = 998244353LL;
typedef long long ll;
typedef unsigned long long ull; 
typedef pair<int, int> pii;
typedef pair<long long, long long> pll;
template<class T> inline bool chmax(T &a, T b) { if (a < b) { a = b; return true;} return false; }
template<class T> inline bool chmin(T &a, T b) { if (a > b) { a = b; return true;} return false; }
///////////////////////////////////////////////////////////////////////////////////////////////////

#define EPS (1e-10)
int N;
double dp[300][300][300];
double func(int a, int b, int c) {
	if (a+b+c==0) return 0;
	if (dp[a][b][c] > EPS) return dp[a][b][c];
	dp[a][b][c] = N;
	if (a) dp[a][b][c] += func(a-1,b+1,c)*a;
	if (b) dp[a][b][c] += func(a,b-1,c+1)*b;
	if (c) dp[a][b][c] += func(a,b,c-1)*c;
	dp[a][b][c] /= (a+b+c);
	return dp[a][b][c]; 
}
int main() {
    ios::sync_with_stdio(false); cin.tie(nullptr);
	cin >> N;
	int a = 0, b = 0, c = 0;
	rep(i,N) {
		int A; cin >> A;
		if (A==0) a++;
		if (A==1) b++;
		if (A==2) c++;
	}

	cout << fixed << setprecision(10) << func(a,b,c) << ln;
}