#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  double arr[N];
  double total;
  cin >> N;
  total = 0;
  for (int i = 0; i < N; i++) {
    cin >> arr[i];
    total = total + 1 / (arr[i]);
  }
  total = 1 / (total);
  cout << total << endl;
}
