import java.util.*;
public class Main{
  public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    float num = 0;
    int i;
    float total = 0;
    for(i=1;i<=n;i++){
      num = (1/(sc.nextFloat()));
      total = total + num;
    }
    System.out.println(1/total);
  }
} import java.util.*; util.* java public class Main{
  public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    float num = 0;
    int i;
    float total = 0;
    for(i=1;i<=n;i++){
      num = (1/(sc.nextFloat()));
      total = total + num;
    }
    System.out.println(1/total);
  }
} Main public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    float num = 0;
    int i;
    float total = 0;
    for(i=1;i<=n;i++){
      num = (1/(sc.nextFloat()));
      total = total + num;
    }
    System.out.println(1/total);
  } main {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    float num = 0;
    int i;
    float total = 0;
    for(i=1;i<=n;i++){
      num = (1/(sc.nextFloat()));
      total = total + num;
    }
    System.out.println(1/total);
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc float num = 0; num 0 int i; i float total = 0; total 0 for(i=1;i<=n;i++){
      num = (1/(sc.nextFloat()));
      total = total + num;
    } i=1; i=1 i 1 i<=n i n i++ i++ i {
      num = (1/(sc.nextFloat()));
      total = total + num;
    } {
      num = (1/(sc.nextFloat()));
      total = total + num;
    } num = (1/(sc.nextFloat())) num (1/(sc.nextFloat())) 1 (sc.nextFloat()) sc.nextFloat sc total = total + num total total + num total num System.out.println(1/total) System.out.println System.out System System.out 1/total 1 total String args[] args public class Main{
  public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    float num = 0;
    int i;
    float total = 0;
    for(i=1;i<=n;i++){
      num = (1/(sc.nextFloat()));
      total = total + num;
    }
    System.out.println(1/total);
  }
} public class Main{
  public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    float num = 0;
    int i;
    float total = 0;
    for(i=1;i<=n;i++){
      num = (1/(sc.nextFloat()));
      total = total + num;
    }
    System.out.println(1/total);
  }
} Main
import java.util.*;
public class Main{
  public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    float num = 0;
    int i;
    float total = 0;
    for(i=1;i<=n;i++){
      num = (1/(sc.nextFloat()));
      total = total + num;
    }
    System.out.println(1/total);
  }
}