
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        for(int i=0;i<n;i++){
            a[i] = sc.nextInt();
        }
        double[] b = new double[n];
        for(int i=0;i<n;i++){
            b[i] = (double)1.0/(double)a[i];
        }
        double c = 0;
        for(int i=0;i<n;i++){
            c += b[i];
        }
        System.out.println(1.0/c);
    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        for(int i=0;i<n;i++){
            a[i] = sc.nextInt();
        }
        double[] b = new double[n];
        for(int i=0;i<n;i++){
            b[i] = (double)1.0/(double)a[i];
        }
        double c = 0;
        for(int i=0;i<n;i++){
            c += b[i];
        }
        System.out.println(1.0/c);
    }
} Main public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        for(int i=0;i<n;i++){
            a[i] = sc.nextInt();
        }
        double[] b = new double[n];
        for(int i=0;i<n;i++){
            b[i] = (double)1.0/(double)a[i];
        }
        double c = 0;
        for(int i=0;i<n;i++){
            c += b[i];
        }
        System.out.println(1.0/c);
    } main {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        for(int i=0;i<n;i++){
            a[i] = sc.nextInt();
        }
        double[] b = new double[n];
        for(int i=0;i<n;i++){
            b[i] = (double)1.0/(double)a[i];
        }
        double c = 0;
        for(int i=0;i<n;i++){
            c += b[i];
        }
        System.out.println(1.0/c);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int[] a = new int[n]; a new int[n] n for(int i=0;i<n;i++){
            a[i] = sc.nextInt();
        } int i=0; int i=0; i 0 i<n i n i++ i++ i {
            a[i] = sc.nextInt();
        } {
            a[i] = sc.nextInt();
        } a[i] = sc.nextInt() a[i] a i sc.nextInt() sc.nextInt sc double[] b = new double[n]; b new double[n] n for(int i=0;i<n;i++){
            b[i] = (double)1.0/(double)a[i];
        } int i=0; int i=0; i 0 i<n i n i++ i++ i {
            b[i] = (double)1.0/(double)a[i];
        } {
            b[i] = (double)1.0/(double)a[i];
        } b[i] = (double)1.0/(double)a[i] b[i] b i (double)1.0/(double)a[i] (double)1.0 (double)a[i] a i double c = 0; c 0 for(int i=0;i<n;i++){
            c += b[i];
        } int i=0; int i=0; i 0 i<n i n i++ i++ i {
            c += b[i];
        } {
            c += b[i];
        } c += b[i] c b[i] b i System.out.println(1.0/c) System.out.println System.out System System.out 1.0/c 1.0 c String[] args args public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        for(int i=0;i<n;i++){
            a[i] = sc.nextInt();
        }
        double[] b = new double[n];
        for(int i=0;i<n;i++){
            b[i] = (double)1.0/(double)a[i];
        }
        double c = 0;
        for(int i=0;i<n;i++){
            c += b[i];
        }
        System.out.println(1.0/c);
    }
} public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        for(int i=0;i<n;i++){
            a[i] = sc.nextInt();
        }
        double[] b = new double[n];
        for(int i=0;i<n;i++){
            b[i] = (double)1.0/(double)a[i];
        }
        double c = 0;
        for(int i=0;i<n;i++){
            c += b[i];
        }
        System.out.println(1.0/c);
    }
} Main

import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        for(int i=0;i<n;i++){
            a[i] = sc.nextInt();
        }
        double[] b = new double[n];
        for(int i=0;i<n;i++){
            b[i] = (double)1.0/(double)a[i];
        }
        double c = 0;
        for(int i=0;i<n;i++){
            c += b[i];
        }
        System.out.println(1.0/c);
    }
}