import java.util.*;
import java.util.ArrayDeque;
import java.util.Queue;
import java.math.RoundingMode;
import java.math.BigDecimal;


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


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