import java.util.*;
 
public class Main {
	public static void main (String[] args) {
		Scanner in = new Scanner(System.in);
		
		int a = in.nextInt();
        double nam = 0;
      
      for(int i = 0;i < a ;i++){
      nam += 1.0/in.nextDouble();
      }
      System.out.print(1.0/nam);
	}
} import java.util.*; util.* java public class Main {
	public static void main (String[] args) {
		Scanner in = new Scanner(System.in);
		
		int a = in.nextInt();
        double nam = 0;
      
      for(int i = 0;i < a ;i++){
      nam += 1.0/in.nextDouble();
      }
      System.out.print(1.0/nam);
	}
} Main public static void main (String[] args) {
		Scanner in = new Scanner(System.in);
		
		int a = in.nextInt();
        double nam = 0;
      
      for(int i = 0;i < a ;i++){
      nam += 1.0/in.nextDouble();
      }
      System.out.print(1.0/nam);
	} main {
		Scanner in = new Scanner(System.in);
		
		int a = in.nextInt();
        double nam = 0;
      
      for(int i = 0;i < a ;i++){
      nam += 1.0/in.nextDouble();
      }
      System.out.print(1.0/nam);
	} Scanner in = new Scanner(System.in); in new Scanner(System.in) int a = in.nextInt(); a in.nextInt() in.nextInt in double nam = 0; nam 0 for(int i = 0;i < a ;i++){
      nam += 1.0/in.nextDouble();
      } int i = 0; int i = 0; i 0 i < a i a i++ i++ i {
      nam += 1.0/in.nextDouble();
      } {
      nam += 1.0/in.nextDouble();
      } nam += 1.0/in.nextDouble() nam 1.0/in.nextDouble() 1.0 in.nextDouble() in.nextDouble in System.out.print(1.0/nam) System.out.print System.out System System.out 1.0/nam 1.0 nam String[] args args public class Main {
	public static void main (String[] args) {
		Scanner in = new Scanner(System.in);
		
		int a = in.nextInt();
        double nam = 0;
      
      for(int i = 0;i < a ;i++){
      nam += 1.0/in.nextDouble();
      }
      System.out.print(1.0/nam);
	}
} public class Main {
	public static void main (String[] args) {
		Scanner in = new Scanner(System.in);
		
		int a = in.nextInt();
        double nam = 0;
      
      for(int i = 0;i < a ;i++){
      nam += 1.0/in.nextDouble();
      }
      System.out.print(1.0/nam);
	}
} Main
import java.util.*;
 
public class Main {
	public static void main (String[] args) {
		Scanner in = new Scanner(System.in);
		
		int a = in.nextInt();
        double nam = 0;
      
      for(int i = 0;i < a ;i++){
      nam += 1.0/in.nextDouble();
      }
      System.out.print(1.0/nam);
	}
}