#include <bits/stdc++.h>
using std::cerr;
using std::cin;
using std::cout;
using std::endl;
using std::string;
using std::vector;
void funcA() {
  int a;
  string s;
  cin >> a;
  cin >> s;
  if (a >= 3200) {
    cout << s << endl;
    ;
  } else {
    cout << "red" << endl;
  }
}
void func() {
  int N;
  cin >> N;
  double x = 0.0;
  for (int i = 0; i < N; ++i) {
    int a;
    cin >> a;
    x += 1.0 / (double)a;
  }
  double y = 1.0 / x;
  printf("%.16f\n", y);
}
int main() { func(); }
