import java.util.*;
 
public class Main {
 
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        double total = 0.0;
        for (int i = 0; i < N; i++) {
        	total += 1.0/sc.nextDouble();
        }
        System.out.println(1.0/total);
    }
} 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 total = 0.0;
        for (int i = 0; i < N; i++) {
        	total += 1.0/sc.nextDouble();
        }
        System.out.println(1.0/total);
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        double total = 0.0;
        for (int i = 0; i < N; i++) {
        	total += 1.0/sc.nextDouble();
        }
        System.out.println(1.0/total);
    } main {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        double total = 0.0;
        for (int i = 0; i < N; i++) {
        	total += 1.0/sc.nextDouble();
        }
        System.out.println(1.0/total);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc double total = 0.0; total 0.0 for (int i = 0; i < N; i++) {
        	total += 1.0/sc.nextDouble();
        } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
        	total += 1.0/sc.nextDouble();
        } {
        	total += 1.0/sc.nextDouble();
        } total += 1.0/sc.nextDouble() total 1.0/sc.nextDouble() 1.0 sc.nextDouble() sc.nextDouble sc System.out.println(1.0/total) System.out.println System.out System System.out 1.0/total 1.0 total String[] args args public class Main {
 
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        double total = 0.0;
        for (int i = 0; i < N; i++) {
        	total += 1.0/sc.nextDouble();
        }
        System.out.println(1.0/total);
    }
} public class Main {
 
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        double total = 0.0;
        for (int i = 0; i < N; i++) {
        	total += 1.0/sc.nextDouble();
        }
        System.out.println(1.0/total);
    }
} Main
import java.util.*;
 
public class Main {
 
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        double total = 0.0;
        for (int i = 0; i < N; i++) {
        	total += 1.0/sc.nextDouble();
        }
        System.out.println(1.0/total);
    }
}