import java.util.Scanner;
public class Main {
    public static void main(String[] args){
    
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int[]a = new int[n];

    double all = 0;
    for(int i=0; i<n; i++) {
    	a[i] = sc.nextInt();
    	all += (double)1.0/a[i];
    }   	
     	System.out.println(1.0/all);
    }
} 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();
    int[]a = new int[n];

    double all = 0;
    for(int i=0; i<n; i++) {
    	a[i] = sc.nextInt();
    	all += (double)1.0/a[i];
    }   	
     	System.out.println(1.0/all);
    }
} Main public static void main(String[] args){
    
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int[]a = new int[n];

    double all = 0;
    for(int i=0; i<n; i++) {
    	a[i] = sc.nextInt();
    	all += (double)1.0/a[i];
    }   	
     	System.out.println(1.0/all);
    } main {
    
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int[]a = new int[n];

    double all = 0;
    for(int i=0; i<n; i++) {
    	a[i] = sc.nextInt();
    	all += (double)1.0/a[i];
    }   	
     	System.out.println(1.0/all);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int[]a = new int[n]; a new int[n] n double all = 0; all 0 for(int i=0; i<n; i++) {
    	a[i] = sc.nextInt();
    	all += (double)1.0/a[i];
    } int i=0; int i=0; i 0 i<n i n i++ i++ i {
    	a[i] = sc.nextInt();
    	all += (double)1.0/a[i];
    } {
    	a[i] = sc.nextInt();
    	all += (double)1.0/a[i];
    } a[i] = sc.nextInt() a[i] a i sc.nextInt() sc.nextInt sc all += (double)1.0/a[i] all (double)1.0/a[i] (double)1.0 a[i] a i System.out.println(1.0/all) System.out.println System.out System System.out 1.0/all 1.0 all String[] args args public class Main {
    public static void main(String[] args){
    
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int[]a = new int[n];

    double all = 0;
    for(int i=0; i<n; i++) {
    	a[i] = sc.nextInt();
    	all += (double)1.0/a[i];
    }   	
     	System.out.println(1.0/all);
    }
} public class Main {
    public static void main(String[] args){
    
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int[]a = new int[n];

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

    double all = 0;
    for(int i=0; i<n; i++) {
    	a[i] = sc.nextInt();
    	all += (double)1.0/a[i];
    }   	
     	System.out.println(1.0/all);
    }
}