import java.util.Scanner;

public class Main{
	 public static void main(String[] args) {
		 float result=0;
	        Scanner scan = new Scanner(System.in);
	        float array[]=new float[scan.nextInt()];
	        for(int i=0;i<array.length;i++){
	        	array[i]=scan.nextInt();
	        	result=result+(1/array[i]);
	        }
	      System.out.print(1/result);
	    }
	} import java.util.Scanner; Scanner java.util public class Main{
	 public static void main(String[] args) {
		 float result=0;
	        Scanner scan = new Scanner(System.in);
	        float array[]=new float[scan.nextInt()];
	        for(int i=0;i<array.length;i++){
	        	array[i]=scan.nextInt();
	        	result=result+(1/array[i]);
	        }
	      System.out.print(1/result);
	    }
	} Main public static void main(String[] args) {
		 float result=0;
	        Scanner scan = new Scanner(System.in);
	        float array[]=new float[scan.nextInt()];
	        for(int i=0;i<array.length;i++){
	        	array[i]=scan.nextInt();
	        	result=result+(1/array[i]);
	        }
	      System.out.print(1/result);
	    } main {
		 float result=0;
	        Scanner scan = new Scanner(System.in);
	        float array[]=new float[scan.nextInt()];
	        for(int i=0;i<array.length;i++){
	        	array[i]=scan.nextInt();
	        	result=result+(1/array[i]);
	        }
	      System.out.print(1/result);
	    } float result=0; result 0 Scanner scan = new Scanner(System.in); scan new Scanner(System.in) float array[]=new float[scan.nextInt()]; array new float[scan.nextInt()] scan.nextInt() scan.nextInt scan for(int i=0;i<array.length;i++){
	        	array[i]=scan.nextInt();
	        	result=result+(1/array[i]);
	        } int i=0; int i=0; i 0 i<array.length i array.length array array.length i++ i++ i {
	        	array[i]=scan.nextInt();
	        	result=result+(1/array[i]);
	        } {
	        	array[i]=scan.nextInt();
	        	result=result+(1/array[i]);
	        } array[i]=scan.nextInt() array[i] array i scan.nextInt() scan.nextInt scan result=result+(1/array[i]) result result+(1/array[i]) result (1/array[i]) 1 array[i] array i System.out.print(1/result) System.out.print System.out System System.out 1/result 1 result String[] args args public class Main{
	 public static void main(String[] args) {
		 float result=0;
	        Scanner scan = new Scanner(System.in);
	        float array[]=new float[scan.nextInt()];
	        for(int i=0;i<array.length;i++){
	        	array[i]=scan.nextInt();
	        	result=result+(1/array[i]);
	        }
	      System.out.print(1/result);
	    }
	} public class Main{
	 public static void main(String[] args) {
		 float result=0;
	        Scanner scan = new Scanner(System.in);
	        float array[]=new float[scan.nextInt()];
	        for(int i=0;i<array.length;i++){
	        	array[i]=scan.nextInt();
	        	result=result+(1/array[i]);
	        }
	      System.out.print(1/result);
	    }
	} Main
import java.util.Scanner;

public class Main{
	 public static void main(String[] args) {
		 float result=0;
	        Scanner scan = new Scanner(System.in);
	        float array[]=new float[scan.nextInt()];
	        for(int i=0;i<array.length;i++){
	        	array[i]=scan.nextInt();
	        	result=result+(1/array[i]);
	        }
	      System.out.print(1/result);
	    }
	}