import java.util.Scanner;
public class Main {
	public static void main(String... args){
      Scanner sc = new Scanner(System.in);
      int N = sc.nextInt();
      double sum = 0.0;
      for(int i = 0; i < N; i++) {
        int input = sc.nextInt();
        sum += (double) (1 / (double)input);
      }
      sum = 1 / sum;
      System.out.println(sum);
      
    }
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String... args){
      Scanner sc = new Scanner(System.in);
      int N = sc.nextInt();
      double sum = 0.0;
      for(int i = 0; i < N; i++) {
        int input = sc.nextInt();
        sum += (double) (1 / (double)input);
      }
      sum = 1 / sum;
      System.out.println(sum);
      
    }
} Main public static void main(String... args){
      Scanner sc = new Scanner(System.in);
      int N = sc.nextInt();
      double sum = 0.0;
      for(int i = 0; i < N; i++) {
        int input = sc.nextInt();
        sum += (double) (1 / (double)input);
      }
      sum = 1 / sum;
      System.out.println(sum);
      
    } main {
      Scanner sc = new Scanner(System.in);
      int N = sc.nextInt();
      double sum = 0.0;
      for(int i = 0; i < N; i++) {
        int input = sc.nextInt();
        sum += (double) (1 / (double)input);
      }
      sum = 1 / sum;
      System.out.println(sum);
      
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc double sum = 0.0; sum 0.0 for(int i = 0; i < N; i++) {
        int input = sc.nextInt();
        sum += (double) (1 / (double)input);
      } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
        int input = sc.nextInt();
        sum += (double) (1 / (double)input);
      } {
        int input = sc.nextInt();
        sum += (double) (1 / (double)input);
      } int input = sc.nextInt(); input sc.nextInt() sc.nextInt sc sum += (double) (1 / (double)input) sum (double) (1 / (double)input) 1 (double)input sum = 1 / sum sum 1 / sum 1 sum System.out.println(sum) System.out.println System.out System System.out sum String... args args public class Main {
	public static void main(String... args){
      Scanner sc = new Scanner(System.in);
      int N = sc.nextInt();
      double sum = 0.0;
      for(int i = 0; i < N; i++) {
        int input = sc.nextInt();
        sum += (double) (1 / (double)input);
      }
      sum = 1 / sum;
      System.out.println(sum);
      
    }
} public class Main {
	public static void main(String... args){
      Scanner sc = new Scanner(System.in);
      int N = sc.nextInt();
      double sum = 0.0;
      for(int i = 0; i < N; i++) {
        int input = sc.nextInt();
        sum += (double) (1 / (double)input);
      }
      sum = 1 / sum;
      System.out.println(sum);
      
    }
} Main
import java.util.Scanner;
public class Main {
	public static void main(String... args){
      Scanner sc = new Scanner(System.in);
      int N = sc.nextInt();
      double sum = 0.0;
      for(int i = 0; i < N; i++) {
        int input = sc.nextInt();
        sum += (double) (1 / (double)input);
      }
      sum = 1 / sum;
      System.out.println(sum);
      
    }
}