#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
  int n;
  cin >> n;
  vector<double> a(n);
  for (int i = 0; i < (n); i++) cin >> a[i];
  double b = 0.0;
  for (int i = 0; i < (n); i++) b += 1.0 / a[i];
  printf("%.10f\n", 1.0 / b);
  return 0;
}
