import java.util.*;
 
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        double resum = 0;
        for(int i = 0; i < N; i++) {
            resum += 1 / Double.parseDouble(sc.next());
        }
        
        System.out.println((double)(1 / resum));
    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        double resum = 0;
        for(int i = 0; i < N; i++) {
            resum += 1 / Double.parseDouble(sc.next());
        }
        
        System.out.println((double)(1 / resum));
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        double resum = 0;
        for(int i = 0; i < N; i++) {
            resum += 1 / Double.parseDouble(sc.next());
        }
        
        System.out.println((double)(1 / resum));
    } main {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        double resum = 0;
        for(int i = 0; i < N; i++) {
            resum += 1 / Double.parseDouble(sc.next());
        }
        
        System.out.println((double)(1 / resum));
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc double resum = 0; resum 0 for(int i = 0; i < N; i++) {
            resum += 1 / Double.parseDouble(sc.next());
        } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
            resum += 1 / Double.parseDouble(sc.next());
        } {
            resum += 1 / Double.parseDouble(sc.next());
        } resum += 1 / Double.parseDouble(sc.next()) resum 1 / Double.parseDouble(sc.next()) 1 Double.parseDouble(sc.next()) Double.parseDouble Double sc.next() sc.next sc System.out.println((double)(1 / resum)) System.out.println System.out System System.out (double)(1 / resum) 1 resum String[] args args public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        double resum = 0;
        for(int i = 0; i < N; i++) {
            resum += 1 / Double.parseDouble(sc.next());
        }
        
        System.out.println((double)(1 / resum));
    }
} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        double resum = 0;
        for(int i = 0; i < N; i++) {
            resum += 1 / Double.parseDouble(sc.next());
        }
        
        System.out.println((double)(1 / resum));
    }
} Main
import java.util.*;
 
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        double resum = 0;
        for(int i = 0; i < N; i++) {
            resum += 1 / Double.parseDouble(sc.next());
        }
        
        System.out.println((double)(1 / resum));
    }
}