#include <bits/stdc++.h>
using namespace std;
#define REP(i,n) for(int i=0;i<(int)(n);++i)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i)
#define ALL(c) (c).begin(), (c).end()
#define valid(y,x,h,w) (0<=y&&y<h&&0<=x&&x<w)
#define tpl(...) make_tuple(__VA_ARGS__)
const int INF = 0x3f3f3f3f;
const double EPS = 1e-8;
const double PI = acos(-1);
typedef long long ll;
typedef pair<int,int> pii;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
template<typename Ch,typename Tr,typename C,typename=decltype(begin(C()))>basic_ostream<Ch,Tr>& operator<<(basic_ostream<Ch,Tr>&os,
const C& c){os<<'[';for(auto i=begin(c);i!=end(c);++i)os<<(i==begin(c)?"":" ")<<*i;return os<<']';}
template<class S,class T>ostream&operator<<(ostream &o,const pair<S,T>&t){return o<<'('<<t.first<<','<<t.second<<')';}
template<int N,class Tp>void output(ostream&,const Tp&){}
template<int N,class Tp,class,class ...Ts>void output(ostream &o,const Tp&t){if(N)o<<',';o<<get<N>(t);output<N+1,Tp,Ts...>(o,t);}
template<class ...Ts>ostream&operator<<(ostream&o,const tuple<Ts...>&t){o<<'(';output<0,tuple<Ts...>,Ts...>(o,t);return o<<')';}
template<class T>void output(T t,char z=10){if(t<0)t=-t,putchar(45);int c[20];
int k=0;while(t)c[k++]=t%10,t/=10;for(k||(c[k++]=0);k;)putchar(c[--k]^48);putchar(z);}
template<class T>void outputs(T t){output(t);}
template<class S,class ...T>void outputs(S a,T...t){output(a,32);outputs(t...);}
template<class T>void output(T *a,int n){REP(i,n)cout<<a[i]<<(i!=n-1?',':'\n');}
template<class T>void output(T *a,int n,int m){REP(i,n)output(a[i],m);}
template<class T>bool input(T &t){int n=1,c;for(t=0;!isdigit(c=getchar())&&~c&&c-45;);
if(!~c)return 0;for(c-45&&(n=0,t=c^48);isdigit(c=getchar());)t=10*t+c-48;t=n?-t:t;return 1;}
template<class S,class ...T>bool input(S&a,T&...t){input(a);return input(t...);}
template<class T>bool inputs(T *a, int n) { REP(i,n) if(!input(a[i])) return 0; return 1;}

double dp[101][101][101];
vector<double> p;
int n;

double solve(int i, int j, int k) {
  if (dp[i][j][k] >= 0) return dp[i][j][k];
  int S = 0;
  if (i>0) S |= 1<<0;
  if (j>0) S |= 1<<1;
  if (k>0) S |= 1<<2;
  if (S == 0) return 0;
  double p[3] = {(double)i / n, (double)j / n, (double)k / n };
  double pp = 0;
  REP(l,3) {
    if (S>>l&1) pp += p[l];
  }
  // outputs(i,j,k);
  double res = 0;
  REP(l,3) {
    if (S>>l&1) {
      int ii = i, jj = j, kk = k;
      if (l == 0) {
        ii--;
      } else if (l == 1) {
        jj--;
        ii++;
      } else {
        kk--;
        jj++;
      }
      res += (solve(ii,jj,kk) + 1.0 / pp) * (p[l]/pp);
    }
  }
  // outputs(i,j,k);
  // cout << res << " " << pp << endl;
  return dp[i][j][k] = res;
}
int main() {
  while(cin >> n) {
    int A[100];
    int B[4] = {};
    REP(i,n) {
      cin >> A[i];
      chmin(A[i],3);
      B[3-A[i]]++;
    }
    double denom = B[0]+B[1]+B[2]+B[3];
    p = vector<double>({B[1] / denom, B[2] / denom, B[3] / denom});
    memset(dp,-1,sizeof(dp));
    double ans = solve(B[1],B[2],B[3]);
    printf("%.10f\n", ans);
  }
}