import java.util.*;
import java.math.*;
import java.io.*;
import java.lang.*;

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

class io{

    public static void YesNo(boolean f) {
        System.out.println(f?"Yes":"No");
    }
    public static void YESNO(boolean f) {
        System.out.println(f?"YES":"NO");
    }
}

class Algo{
    public static boolean isOdd(int n){
        return n % 2 == 1;
    }
    public static boolean isEven(int n){
        return n % 2 == 0;
    }


}
 import java.util.*; util.* java import java.math.*; math.* java import java.io.*; io.* java import java.lang.*; lang.* java public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] ar = new int[n];
        double res = 0;
        for (int i = 0; i < n; i++) {
            ar[i] = sc.nextInt();
            res += (1/(double)ar[i]);
        }
        System.out.println(1/res);
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] ar = new int[n];
        double res = 0;
        for (int i = 0; i < n; i++) {
            ar[i] = sc.nextInt();
            res += (1/(double)ar[i]);
        }
        System.out.println(1/res);
    } main {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] ar = new int[n];
        double res = 0;
        for (int i = 0; i < n; i++) {
            ar[i] = sc.nextInt();
            res += (1/(double)ar[i]);
        }
        System.out.println(1/res);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int[] ar = new int[n]; ar new int[n] n double res = 0; res 0 for (int i = 0; i < n; i++) {
            ar[i] = sc.nextInt();
            res += (1/(double)ar[i]);
        } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
            ar[i] = sc.nextInt();
            res += (1/(double)ar[i]);
        } {
            ar[i] = sc.nextInt();
            res += (1/(double)ar[i]);
        } ar[i] = sc.nextInt() ar[i] ar i sc.nextInt() sc.nextInt sc res += (1/(double)ar[i]) res (1/(double)ar[i]) 1 (double)ar[i] ar i System.out.println(1/res) System.out.println System.out System System.out 1/res 1 res String[] args args class io{

    public static void YesNo(boolean f) {
        System.out.println(f?"Yes":"No");
    }
    public static void YESNO(boolean f) {
        System.out.println(f?"YES":"NO");
    }
} io public static void YesNo(boolean f) {
        System.out.println(f?"Yes":"No");
    } YesNo {
        System.out.println(f?"Yes":"No");
    } System.out.println(f?"Yes":"No") System.out.println System.out System System.out f?"Yes":"No" f "Yes" "No" boolean f f public static void YESNO(boolean f) {
        System.out.println(f?"YES":"NO");
    } YESNO {
        System.out.println(f?"YES":"NO");
    } System.out.println(f?"YES":"NO") System.out.println System.out System System.out f?"YES":"NO" f "YES" "NO" boolean f f class Algo{
    public static boolean isOdd(int n){
        return n % 2 == 1;
    }
    public static boolean isEven(int n){
        return n % 2 == 0;
    }


} Algo public static boolean isOdd(int n){
        return n % 2 == 1;
    } isOdd {
        return n % 2 == 1;
    } return n % 2 == 1; n % 2 == 1 n % 2 n 2 1 int n n public static boolean isEven(int n){
        return n % 2 == 0;
    } isEven {
        return n % 2 == 0;
    } return n % 2 == 0; n % 2 == 0 n % 2 n 2 0 int n n public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] ar = new int[n];
        double res = 0;
        for (int i = 0; i < n; i++) {
            ar[i] = sc.nextInt();
            res += (1/(double)ar[i]);
        }
        System.out.println(1/res);
    }
} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] ar = new int[n];
        double res = 0;
        for (int i = 0; i < n; i++) {
            ar[i] = sc.nextInt();
            res += (1/(double)ar[i]);
        }
        System.out.println(1/res);
    }
} Main
import java.util.*;
import java.math.*;
import java.io.*;
import java.lang.*;

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

class io{

    public static void YesNo(boolean f) {
        System.out.println(f?"Yes":"No");
    }
    public static void YESNO(boolean f) {
        System.out.println(f?"YES":"NO");
    }
}

class Algo{
    public static boolean isOdd(int n){
        return n % 2 == 1;
    }
    public static boolean isEven(int n){
        return n % 2 == 0;
    }


}

