#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
typedef long long ll;
typedef pair<ll,ll> P;
double dp[310][310][310];
ll n;
double calc(ll i,ll j,ll k){
  double tmp=0;
  if(dp[i][j][k]>=0){
    return dp[i][j][k];
  }
  if(i==j&&j==k&&i==0) return 0.000;
  if(i>0) tmp += calc(i-1,j,k)*i;
  if(j>0) tmp += calc(i+1,j-1,k)*j;
  if(k>0) tmp += calc(i,j+1,k-1)*k;
  tmp += n;
  tmp *= 1.0/(i+j+k);

  return dp[i][j][k] = tmp;
}
signed main(){
  cin >> n;
  ll a[n],on,tw,th;
  on=tw=th=0;
  for(int i=0;i<n;i++){
    cin >> a[i];
    if(a[i]==1) on++;
    else if(a[i]==2) tw++;
    else th++;
  }
  fill(dp[0][0],dp[310][0],-1.0);
  printf("%.15lf\n",calc(on,tw,th));
}
