#include <iostream>
#include <algorithm>
using namespace std;
double Calc(int n){
  double a=0;
  for(int i=1;i<=n;++i){
    double b=1;
    double c=(double)i;
    double d=(b/c);
    a+=d;
  }
  return a;
}
int main(){
  int N;cin>>N;
  double a=Calc(N);
  cerr<<a<<endl;
  int A;
  double b=0;
  for(int i=0;i<N;++i){
    cin>>A;
    int B=min(A,3);
    double C=(double)(3-B);
    double D=(a*C);
    b+=D;
  }
  cout<<b<<endl;
}