import java.util.*;

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

public class Main {
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    double ans = 0;
    double[] ar = new double[n];
    for(int i=0;i<n;i++){
      ar[i] = 1 / sc.nextDouble();
      ans += ar[i];
    }
    System.out.println(1/ans);
  }
}