import java.util.*;

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

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        double bumbo = 0.0;
        for (int i = 0; i < n; i++) {
            int a = sc.nextInt();
            bumbo += 1.0/a;
        }
        double result = 1.0/bumbo;
        System.out.println(result);
    }
}
