import java.util.*;

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

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