import java.util.*;
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();
        Double[] a = new Double[n];
        double b = 1;
        for(int i=0; i< n; i++) {
            a[i] = sc.nextDouble();
            b = b*a[i];
        }
        double mo = 0;
        for(int j=0; j< n; j++){
            mo += b/a[j];
        }
        System.out.println(b/mo);

    }
} 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[] a = new Double[n];
        double b = 1;
        for(int i=0; i< n; i++) {
            a[i] = sc.nextDouble();
            b = b*a[i];
        }
        double mo = 0;
        for(int j=0; j< n; j++){
            mo += b/a[j];
        }
        System.out.println(b/mo);

    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();
        Double[] a = new Double[n];
        double b = 1;
        for(int i=0; i< n; i++) {
            a[i] = sc.nextDouble();
            b = b*a[i];
        }
        double mo = 0;
        for(int j=0; j< n; j++){
            mo += b/a[j];
        }
        System.out.println(b/mo);

    } main {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();
        Double[] a = new Double[n];
        double b = 1;
        for(int i=0; i< n; i++) {
            a[i] = sc.nextDouble();
            b = b*a[i];
        }
        double mo = 0;
        for(int j=0; j< n; j++){
            mo += b/a[j];
        }
        System.out.println(b/mo);

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

        int n = sc.nextInt();
        Double[] a = new Double[n];
        double b = 1;
        for(int i=0; i< n; i++) {
            a[i] = sc.nextDouble();
            b = b*a[i];
        }
        double mo = 0;
        for(int j=0; j< n; j++){
            mo += b/a[j];
        }
        System.out.println(b/mo);

    }
} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();
        Double[] a = new Double[n];
        double b = 1;
        for(int i=0; i< n; i++) {
            a[i] = sc.nextDouble();
            b = b*a[i];
        }
        double mo = 0;
        for(int j=0; j< n; j++){
            mo += b/a[j];
        }
        System.out.println(b/mo);

    }
} Main
import java.util.*;
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();
        Double[] a = new Double[n];
        double b = 1;
        for(int i=0; i< n; i++) {
            a[i] = sc.nextDouble();
            b = b*a[i];
        }
        double mo = 0;
        for(int j=0; j< n; j++){
            mo += b/a[j];
        }
        System.out.println(b/mo);

    }
}
