import java.util.Scanner;

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

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