import java.util.Scanner;

public class Main {
    static Scanner stdIn = new Scanner(System.in);
    public static void main(String[] args){
        int n = stdIn.nextInt();
        long[] a = new long[n];
        for(int i=0; i<n; i++){
            a[i] = stdIn.nextLong();
        }

        double inverseSum = 0.0;
        for(int i=0; i<n; i++){
            inverseSum += ((double)1 / a[i]);
        }

        System.out.println((double)1 / inverseSum);
       }
} import java.util.Scanner; Scanner java.util public class Main {
    static Scanner stdIn = new Scanner(System.in);
    public static void main(String[] args){
        int n = stdIn.nextInt();
        long[] a = new long[n];
        for(int i=0; i<n; i++){
            a[i] = stdIn.nextLong();
        }

        double inverseSum = 0.0;
        for(int i=0; i<n; i++){
            inverseSum += ((double)1 / a[i]);
        }

        System.out.println((double)1 / inverseSum);
       }
} Main static Scanner stdIn = new Scanner(System.in); stdIn new Scanner(System.in) public static void main(String[] args){
        int n = stdIn.nextInt();
        long[] a = new long[n];
        for(int i=0; i<n; i++){
            a[i] = stdIn.nextLong();
        }

        double inverseSum = 0.0;
        for(int i=0; i<n; i++){
            inverseSum += ((double)1 / a[i]);
        }

        System.out.println((double)1 / inverseSum);
       } main {
        int n = stdIn.nextInt();
        long[] a = new long[n];
        for(int i=0; i<n; i++){
            a[i] = stdIn.nextLong();
        }

        double inverseSum = 0.0;
        for(int i=0; i<n; i++){
            inverseSum += ((double)1 / a[i]);
        }

        System.out.println((double)1 / inverseSum);
       } int n = stdIn.nextInt(); n stdIn.nextInt() stdIn.nextInt stdIn long[] a = new long[n]; a new long[n] n for(int i=0; i<n; i++){
            a[i] = stdIn.nextLong();
        } int i=0; int i=0; i 0 i<n i n i++ i++ i {
            a[i] = stdIn.nextLong();
        } {
            a[i] = stdIn.nextLong();
        } a[i] = stdIn.nextLong() a[i] a i stdIn.nextLong() stdIn.nextLong stdIn double inverseSum = 0.0; inverseSum 0.0 for(int i=0; i<n; i++){
            inverseSum += ((double)1 / a[i]);
        } int i=0; int i=0; i 0 i<n i n i++ i++ i {
            inverseSum += ((double)1 / a[i]);
        } {
            inverseSum += ((double)1 / a[i]);
        } inverseSum += ((double)1 / a[i]) inverseSum ((double)1 / a[i]) (double)1 a[i] a i System.out.println((double)1 / inverseSum) System.out.println System.out System System.out (double)1 / inverseSum (double)1 inverseSum String[] args args public class Main {
    static Scanner stdIn = new Scanner(System.in);
    public static void main(String[] args){
        int n = stdIn.nextInt();
        long[] a = new long[n];
        for(int i=0; i<n; i++){
            a[i] = stdIn.nextLong();
        }

        double inverseSum = 0.0;
        for(int i=0; i<n; i++){
            inverseSum += ((double)1 / a[i]);
        }

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

        double inverseSum = 0.0;
        for(int i=0; i<n; i++){
            inverseSum += ((double)1 / a[i]);
        }

        System.out.println((double)1 / inverseSum);
       }
} Main
import java.util.Scanner;

public class Main {
    static Scanner stdIn = new Scanner(System.in);
    public static void main(String[] args){
        int n = stdIn.nextInt();
        long[] a = new long[n];
        for(int i=0; i<n; i++){
            a[i] = stdIn.nextLong();
        }

        double inverseSum = 0.0;
        for(int i=0; i<n; i++){
            inverseSum += ((double)1 / a[i]);
        }

        System.out.println((double)1 / inverseSum);
       }
}