import java.util.*;

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

        int N = sc.nextInt();
        double sum = 0;
        double ans = 0;

        for(int i=0; i<N; i++){
            sum = sum + 1/(double)sc.nextInt();
        }
        ans =1/sum;
        System.out.println(ans);
    }
} 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 sum = 0;
        double ans = 0;

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

        int N = sc.nextInt();
        double sum = 0;
        double ans = 0;

        for(int i=0; i<N; i++){
            sum = sum + 1/(double)sc.nextInt();
        }
        ans =1/sum;
        System.out.println(ans);
    } main {
        Scanner sc = new Scanner(System.in);

        int N = sc.nextInt();
        double sum = 0;
        double ans = 0;

        for(int i=0; i<N; i++){
            sum = sum + 1/(double)sc.nextInt();
        }
        ans =1/sum;
        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 sum = 0; sum 0 double ans = 0; ans 0 for(int i=0; i<N; i++){
            sum = sum + 1/(double)sc.nextInt();
        } int i=0; int i=0; i 0 i<N i N i++ i++ i {
            sum = sum + 1/(double)sc.nextInt();
        } {
            sum = sum + 1/(double)sc.nextInt();
        } sum = sum + 1/(double)sc.nextInt() sum sum + 1/(double)sc.nextInt() sum 1/(double)sc.nextInt() 1 (double)sc.nextInt() sc.nextInt sc ans =1/sum ans 1/sum 1 sum 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 sum = 0;
        double ans = 0;

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

        int N = sc.nextInt();
        double sum = 0;
        double ans = 0;

        for(int i=0; i<N; i++){
            sum = sum + 1/(double)sc.nextInt();
        }
        ans =1/sum;
        System.out.println(ans);
    }
} Main
import java.util.*;

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

        int N = sc.nextInt();
        double sum = 0;
        double ans = 0;

        for(int i=0; i<N; i++){
            sum = sum + 1/(double)sc.nextInt();
        }
        ans =1/sum;
        System.out.println(ans);
    }
}