import java.util.*;
import java.math.BigDecimal;
 
public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    double ary[] = new double[n];
    double ans = 0;
    
    for (int i = 0; i < n; i++) { 
      ary[i] = sc.nextDouble(); 
      ans += 1 / ary[i];
    }
    
    ans = 1 / 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