#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  double a = 0;
  cin >> n;
  for (int i = 0; i < n; i++) {
    double x;
    cin >> x;
    a += (1 / x);
  }
  printf("%.5f\n", (double)1 / a);
}
