import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 入力
		int a = sc.nextInt();
	    int b;
      	double result = 0.0;
		for (int i = 0; i < a; i++){
          	b = sc.nextInt();
      		result =result + (1.0 / b);
	    }
      System.out.println(1.0 / result);
	}
} import java.util.*; util.* java public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 入力
		int a = sc.nextInt();
	    int b;
      	double result = 0.0;
		for (int i = 0; i < a; i++){
          	b = sc.nextInt();
      		result =result + (1.0 / b);
	    }
      System.out.println(1.0 / result);
	}
} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 入力
		int a = sc.nextInt();
	    int b;
      	double result = 0.0;
		for (int i = 0; i < a; i++){
          	b = sc.nextInt();
      		result =result + (1.0 / b);
	    }
      System.out.println(1.0 / result);
	} main {
		Scanner sc = new Scanner(System.in);
		// 入力
		int a = sc.nextInt();
	    int b;
      	double result = 0.0;
		for (int i = 0; i < a; i++){
          	b = sc.nextInt();
      		result =result + (1.0 / b);
	    }
      System.out.println(1.0 / result);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int a = sc.nextInt(); a sc.nextInt() sc.nextInt sc int b; b double result = 0.0; result 0.0 for (int i = 0; i < a; i++){
          	b = sc.nextInt();
      		result =result + (1.0 / b);
	    } int i = 0; int i = 0; i 0 i < a i a i++ i++ i {
          	b = sc.nextInt();
      		result =result + (1.0 / b);
	    } {
          	b = sc.nextInt();
      		result =result + (1.0 / b);
	    } b = sc.nextInt() b sc.nextInt() sc.nextInt sc result =result + (1.0 / b) result result + (1.0 / b) result (1.0 / b) 1.0 b System.out.println(1.0 / result) System.out.println System.out System System.out 1.0 / result 1.0 result String[] args args public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 入力
		int a = sc.nextInt();
	    int b;
      	double result = 0.0;
		for (int i = 0; i < a; i++){
          	b = sc.nextInt();
      		result =result + (1.0 / b);
	    }
      System.out.println(1.0 / result);
	}
} public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 入力
		int a = sc.nextInt();
	    int b;
      	double result = 0.0;
		for (int i = 0; i < a; i++){
          	b = sc.nextInt();
      		result =result + (1.0 / b);
	    }
      System.out.println(1.0 / result);
	}
} Main
import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 入力
		int a = sc.nextInt();
	    int b;
      	double result = 0.0;
		for (int i = 0; i < a; i++){
          	b = sc.nextInt();
      		result =result + (1.0 / b);
	    }
      System.out.println(1.0 / result);
	}
}