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