import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scan=new Scanner(System.in);
        int n=scan.nextInt();
        int a;
        double b=0;
        double ans;

        for(int i=0;i<n;i++){
            a=scan.nextInt();
            b+=1.0/a;
//            System.out.println(b);
        }

        ans=1/b;
        System.out.println(ans);
    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        Scanner scan=new Scanner(System.in);
        int n=scan.nextInt();
        int a;
        double b=0;
        double ans;

        for(int i=0;i<n;i++){
            a=scan.nextInt();
            b+=1.0/a;
//            System.out.println(b);
        }

        ans=1/b;
        System.out.println(ans);
    }
} Main public static void main(String[] args) {
        Scanner scan=new Scanner(System.in);
        int n=scan.nextInt();
        int a;
        double b=0;
        double ans;

        for(int i=0;i<n;i++){
            a=scan.nextInt();
            b+=1.0/a;
//            System.out.println(b);
        }

        ans=1/b;
        System.out.println(ans);
    } main {
        Scanner scan=new Scanner(System.in);
        int n=scan.nextInt();
        int a;
        double b=0;
        double ans;

        for(int i=0;i<n;i++){
            a=scan.nextInt();
            b+=1.0/a;
//            System.out.println(b);
        }

        ans=1/b;
        System.out.println(ans);
    } Scanner scan=new Scanner(System.in); scan new Scanner(System.in) int n=scan.nextInt(); n scan.nextInt() scan.nextInt scan int a; a double b=0; b 0 double ans; ans for(int i=0;i<n;i++){
            a=scan.nextInt();
            b+=1.0/a;
//            System.out.println(b);
        } int i=0; int i=0; i 0 i<n i n i++ i++ i {
            a=scan.nextInt();
            b+=1.0/a;
//            System.out.println(b);
        } {
            a=scan.nextInt();
            b+=1.0/a;
//            System.out.println(b);
        } a=scan.nextInt() a scan.nextInt() scan.nextInt scan b+=1.0/a b 1.0/a 1.0 a ans=1/b ans 1/b 1 b System.out.println(ans) System.out.println System.out System System.out ans String[] args args public class Main {
    public static void main(String[] args) {
        Scanner scan=new Scanner(System.in);
        int n=scan.nextInt();
        int a;
        double b=0;
        double ans;

        for(int i=0;i<n;i++){
            a=scan.nextInt();
            b+=1.0/a;
//            System.out.println(b);
        }

        ans=1/b;
        System.out.println(ans);
    }
} public class Main {
    public static void main(String[] args) {
        Scanner scan=new Scanner(System.in);
        int n=scan.nextInt();
        int a;
        double b=0;
        double ans;

        for(int i=0;i<n;i++){
            a=scan.nextInt();
            b+=1.0/a;
//            System.out.println(b);
        }

        ans=1/b;
        System.out.println(ans);
    }
} Main
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scan=new Scanner(System.in);
        int n=scan.nextInt();
        int a;
        double b=0;
        double ans;

        for(int i=0;i<n;i++){
            a=scan.nextInt();
            b+=1.0/a;
//            System.out.println(b);
        }

        ans=1/b;
        System.out.println(ans);
    }
}