#include <bits/stdc++.h>
      
#define FOR(i,a,b) for(ll i = (a); i < (ll)(b); i++)
#define REP(i,n) FOR(i,0,n)
#define YYS(x,arr) for(auto& x:arr)
#define PW(x) (1LL<<(x))
#define SZ(x) ((ll)(x).size())

#define pb emplace_back
#define fi first
#define se second

using namespace std;

using ld = long double;
using ll = long long int;

const ll INF = (ll)1e9 + 10;
const ll INFLL = (ll)1e18 + 10;
const ll MOD = 1e9+7;
     
template<class T> T &chmin( T &a , const T &b ){ return a = min(a,b); }
template<class T> T &chmax( T &a , const T &b ){ return a = max(a,b); }
template<class T> void UNIQUE(vector<T> &a){ a.erase(unique(a.begin(), a.end()), a.end()); }

template<class S, class T> ostream& operator << (ostream& os, const pair<S, T> v){
  os << "(" << v.first << ", " << v.second << ")"; return os;
}
template<class T> ostream& operator << (ostream& os, const vector<T> v){
  for(int i = 0; i < v.size(); i++){if(i > 0){os << " ";} os << v[i];} return os;
}
template<class T> ostream& operator << (ostream& os, const vector<vector<T>> v){
  for(int i = 0; i < v.size(); i++){if(i > 0){os << endl;} os << v[i];} return os;
}

ll in(){long long int x; assert(scanf("%lld", &x) == 1); return x;}
ld fin(){double x; assert(scanf("%lf", &x) == 1); return x;}
char yuyushiki[1000010]; string stin(){assert(scanf("%s", yuyushiki) == 1); return yuyushiki;}

// head

struct Mod{
  unsigned n;
  Mod() : n(0){}
  Mod( ll x ){
    if( x < 0 ) n = x%MOD+MOD;
    else n = x%MOD;
  }
};
Mod operator + ( Mod a  , Mod b ){ return Mod( a.n + b.n ); }
Mod operator +=( Mod &a , Mod b ){ return a = a + b; }
Mod operator - ( Mod a ){ return Mod( MOD - a.n ); }
Mod operator - ( Mod a  , Mod b ){ return Mod( a.n + MOD - b.n ); }
Mod operator -=( Mod &a , Mod b ){ return a = a - b; }
Mod operator * ( Mod a  , Mod b ){ return Mod( (ll)a.n * b.n ); }
Mod operator *=( Mod &a , Mod b ){ return a = a * b; }
Mod modpow( Mod x , ll k ){
  Mod res = 1;
  while( k ){
    if( k & 1 ) res *= x;
    k /= 2;
    x *= x;
  }
  return res;
}
ll extgcd( ll a , ll b , ll &x , ll &y ){
  ll d = a;
  if( b != 0 ){
    d = extgcd( b , a % b , y , x );
    y -= a / b * x;
  } else {
    x = 1, y = 0;
  }
  return d;
}
Mod inv( Mod a ){ ll x, y; assert( extgcd( a.n , MOD , x , y ) == 1 ); return Mod( x ); }
Mod operator / ( Mod a  , Mod b ){ return Mod( (ll)a.n * inv(b).n ); }
Mod operator /=( Mod &a , Mod b ){ return a = a / b; }

int a, b, c;
ld dp[310][310][310];

int main(){

  ll n = in();
  REP(i, n){
    int x = in();
    if(x == 1){
      a++;
    } else if(x == 2){
      b++;
    } else if(x == 3){
      c++;
    }
  }

  dp[0][0][0] = 0;
  REP(k, n + 1){
    REP(j, n + 1){
      REP(i, n + 1){
        if(i == 0 && j == 0 && k == 0){
          continue;
        }
        if(i + j + k > n){
          continue;
        }
        ll z = n - i - j - k;
        ld pa = (ld)i / n;
        ld ca = 0;
        if(i > 0){
          ca = pa * dp[i-1][j][k];
        }
        ld pb = (ld)j / n;
        ld cb = 0;
        if(j > 0){
          cb = pb * dp[i+1][j-1][k];
        }
        ld pc = (ld)k / n;
        ld cc = 0;
        if(k > 0){
          cc = pc * dp[i][j+1][k-1];
        }
        ld pz = (ld)z / n;
        dp[i][j][k] = (ca + cb + cc + 1) / (1 - pz);
      }
    }
  }
  
  printf("%.18lf\n", (double)dp[a][b][c]);
  
  return 0;
}


Measured memory_bytes: 450688