#include <bits/stdc++.h>
using namespace std;
int main() {
  double sum = 1, ans = 0;
  int n, x = 1;
  cin >> n;
  int arr[n];
  for (int i = 0; i < n; i++) {
    cin >> arr[i];
    sum += (1 / (arr[i] * 1.0));
  }
  cout << 1 / sum << endl;
  return 0;
}
