#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int num;
  long double sum = 0;
  vector<long double> vect(100010);
  cin >> num;
  for (long long int i = 0; i < num; i++) {
    cin >> vect.at(i);
  }
  for (long long int i = 0; i < num; i++) {
    sum += (long double)1 / vect[i];
  }
  cout << (long double)(1 / sum) << endl;
  return 0;
}
