import java.util.*;

public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        double d = 0.0;
        int n = sc.nextInt();
        for (int i=0;i<n;i++)d+=1/sc.nextDouble();
        System.out.println(1/d);
    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        double d = 0.0;
        int n = sc.nextInt();
        for (int i=0;i<n;i++)d+=1/sc.nextDouble();
        System.out.println(1/d);
    }
} Main public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        double d = 0.0;
        int n = sc.nextInt();
        for (int i=0;i<n;i++)d+=1/sc.nextDouble();
        System.out.println(1/d);
    } main {
        Scanner sc = new Scanner(System.in);
        double d = 0.0;
        int n = sc.nextInt();
        for (int i=0;i<n;i++)d+=1/sc.nextDouble();
        System.out.println(1/d);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) double d = 0.0; d 0.0 int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc for (int i=0;i<n;i++)d+=1/sc.nextDouble(); int i=0; int i=0; i 0 i<n i n i++ i++ i d+=1/sc.nextDouble(); d+=1/sc.nextDouble() d 1/sc.nextDouble() 1 sc.nextDouble() sc.nextDouble sc System.out.println(1/d) System.out.println System.out System System.out 1/d 1 d String[] args args public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        double d = 0.0;
        int n = sc.nextInt();
        for (int i=0;i<n;i++)d+=1/sc.nextDouble();
        System.out.println(1/d);
    }
} public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        double d = 0.0;
        int n = sc.nextInt();
        for (int i=0;i<n;i++)d+=1/sc.nextDouble();
        System.out.println(1/d);
    }
} Main
import java.util.*;

public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        double d = 0.0;
        int n = sc.nextInt();
        for (int i=0;i<n;i++)d+=1/sc.nextDouble();
        System.out.println(1/d);
    }
}
