import java.util.*;
import java.io.*;

class Main {
  public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    int n = scan.nextInt();

    List<Integer> a = new ArrayList<Integer>();
    for(int i = 0; i < n; i++) {
      a.add(scan.nextInt());
    }

    double ans = 0.0;
    for(int i : a) ans += 1.0 / i;
    ans = 1.0 / ans;

    System.out.println(ans);
  }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz