#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  double t = 0.;
  cin >> N;
  int a[N];
  for (int i = 0; i < N; i++) {
    int A;
    cin >> A;
    a[i] = A;
  }
  for (int i = 0; i < N; i++) {
    t = t + 1 / a[i];
  }
  cout << 1 / t << endl;
  return 0;
}
