import java.util.*;

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

public class Main{
    public static void main(String args[]){
        Scanner scanner = new Scanner(System.in);
        int N = scanner.nextInt();
        double sum = 0.0;
        for(int i=0; i<N; i++){
            int A = scanner.nextInt();
            sum += 1.0/A;
        }
        System.out.println(1.0/sum);
    }
}