#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  cout.precision(12);
  cout << fixed;
  int n;
  double s = 0, o = 1;
  cin >> n;
  for (int i = 0; i < n; i++) {
    int k;
    cin >> k;
    s += o / k;
  }
  cout << o / s;
}
