#include <bits/stdc++.h>
//#include <atcoder/all>
//using namespace atcoder;
#pragma GCC target ("avx2")
#pragma GCC optimization ("O3")
#pragma GCC optimize("Ofast")
#pragma GCC optimization ("unroll-loops")
 
using namespace std;
 
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<int, int> pii;
typedef pair<long long, long long> PLL;
typedef pair<int, PII> TIII;
 
typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
 
 
#define FOR(i, s, n) for (int i = s; i < (int)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define rep(i, a, b) for (int i = a; i < (b); ++i)
#define trav(a, x) for (auto &a : x)
#define all(x) x.begin(), x.end()
 
 
template<class T1, class T2> inline bool chmax(T1 &a, T2 b) {if (a < b) {a = b; return true;} return false;}
template<class T1, class T2> inline bool chmin(T1 &a, T2 b) {if (a > b) {a = b; return true;} return false;}
const double EPS = 1e-12, PI = acos(-1);
const double pi = 3.141592653589793238462643383279;
//ここから編集    
typedef string::const_iterator State;
ll GCD(ll a, ll b){
  return (b==0)?a:GCD(b, a%b);
}
ll LCM(ll a, ll b){
  return a/GCD(a, b) * b;
}
template< int mod >
struct ModInt {
  int x;
 
  ModInt() : x(0) {}
  ModInt(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}
 
  ModInt &operator+=(const ModInt &p) {
    if((x += p.x) >= mod) x -= mod;
    return *this;
  }
 
  ModInt &operator-=(const ModInt &p) {
    if((x += mod - p.x) >= mod) x -= mod;
    return *this;
  }
 
  ModInt &operator*=(const ModInt &p) {
    x = (int) (1LL * x * p.x % mod);
    return *this;
  }
 
  ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }
 
  ModInt operator-() const { return ModInt(-x); }
 
  ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }
 
  ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }
 
  ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }
 
  ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }
 
  bool operator==(const ModInt &p) const { return x == p.x; }
 
  bool operator!=(const ModInt &p) const { return x != p.x; }
 
  ModInt inverse() const {
    int a = x, b = mod, u = 1, v = 0, t;
    while(b > 0) {
      t = a / b;
      swap(a -= t * b, b);
      swap(u -= t * v, v);
    }
    return ModInt(u);
  }
 
  ModInt pow(int64_t n) const {
    ModInt ret(1), mul(x);
    while(n > 0) {
      if(n & 1) ret *= mul;
      mul *= mul;
      n >>= 1;
    }
    return ret;
  }
 
  friend ostream &operator<<(ostream &os, const ModInt &p) {
    return os << p.x;
  }
 
  friend istream &operator>>(istream &is, ModInt &a) {
    int64_t t;
    is >> t;
    a = ModInt< mod >(t);
    return (is);
  }
 
  static int get_mod() { return mod; }
};
 
using modint = ModInt< 1000000007 >;
using modint9 = ModInt<998244353>;
template< typename T >
struct Combination {
  vector< T > _fact, _rfact, _inv;
 
  Combination(int sz) : _fact(sz + 1), _rfact(sz + 1), _inv(sz + 1) {
    _fact[0] = _rfact[sz] = _inv[0] = 1;
    for(int i = 1; i <= sz; i++) _fact[i] = _fact[i - 1] * i;
    _rfact[sz] /= _fact[sz];
    for(int i = sz - 1; i >= 0; i--) _rfact[i] = _rfact[i + 1] * (i + 1);
    for(int i = 1; i <= sz; i++) _inv[i] = _rfact[i] * _fact[i - 1];
  }
 
  inline T fact(int k) const { return _fact[k]; }
 
  inline T rfact(int k) const { return _rfact[k]; }
 
  inline T inv(int k) const { return _inv[k]; }
 
  T P(int n, int r) const {
    if(r < 0 || n < r) return 0;
    return fact(n) * rfact(n - r);
  }
 
  T C(int p, int q) const {
    if(q < 0 || p < q) return 0;
    return fact(p) * rfact(q) * rfact(p - q);
  }
 
  T H(int n, int r) const {
    if(n < 0 || r < 0) return (0);
    return r == 0 ? 1 : C(n + r - 1, r);
  }
};

ll modpow(ll x, ll n, ll mod = 100000007) {
  ll res = 1;
  while(n) {
    if(n&1) res = (res * x)%mod;
    x = (x*x)%mod;
    n >>= 1;
  }
  return res;
}
int n; 
double dp[110][110][110];
double rec(int a, int b, int c) {
  if(dp[a][b][c] != -1.0) return dp[a][b][c];
  double res = 0.0;
  
  double p = (double)(n-c)/n;
  if(a != n) res += (rec(a+1, b, c) + 1.0/p) * (n-a) / (n-c);
  if(a-b != 0) res += (rec(a, b+1, c) + 1.0/p) * (a-b) / (n-c);
  if(b-c != 0) res += (rec(a, b, c+1) + 1.0/p) * (b-c) / (n-c);
  return dp[a][b][c] = res;  
}
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(12);
  
  cin >> n;
  vector<int> A(n);
  int a=0, b=0, c=0;
  REP(i,n) {
    cin >> A[i];
    if(A[i] == 0) a++;
    if(A[i] == 1) b++;
    if(A[i] == 2) c++;
  }
  REP(i,n+1) REP(j,n+1) REP(k,n+1) dp[i][j][k] = -1.0;
  dp[n][n][n] = 0;

  cout << rec(n-a, n-(a+b), n-(a+b+c)) << endl;
  return 0;
}