import java.util.*;
import java.io.*;
public class Main {
	
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        OutputStream pr = System.out;
        PrintWriter out = new PrintWriter(pr);
        int n = sc.nextInt();
        double d =0.0;
        for(int i=0;i<n;i++){
            double val = sc.nextDouble();
            double ans = 1/val;
            d+=ans;
        }
        d=1/d;
        out.print(d);
        out.close();
    }
    public static long gcd(long a,long b){
        if(a==0){
            return b;
        }else{
            return gcd(b%a,a);
        }
    }
} import java.util.*; util.* java import java.io.*; io.* java public class Main {
	
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        OutputStream pr = System.out;
        PrintWriter out = new PrintWriter(pr);
        int n = sc.nextInt();
        double d =0.0;
        for(int i=0;i<n;i++){
            double val = sc.nextDouble();
            double ans = 1/val;
            d+=ans;
        }
        d=1/d;
        out.print(d);
        out.close();
    }
    public static long gcd(long a,long b){
        if(a==0){
            return b;
        }else{
            return gcd(b%a,a);
        }
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        OutputStream pr = System.out;
        PrintWriter out = new PrintWriter(pr);
        int n = sc.nextInt();
        double d =0.0;
        for(int i=0;i<n;i++){
            double val = sc.nextDouble();
            double ans = 1/val;
            d+=ans;
        }
        d=1/d;
        out.print(d);
        out.close();
    } main {
        Scanner sc = new Scanner(System.in);
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        OutputStream pr = System.out;
        PrintWriter out = new PrintWriter(pr);
        int n = sc.nextInt();
        double d =0.0;
        for(int i=0;i<n;i++){
            double val = sc.nextDouble();
            double ans = 1/val;
            d+=ans;
        }
        d=1/d;
        out.print(d);
        out.close();
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); br new BufferedReader(new InputStreamReader(System.in)) OutputStream pr = System.out; pr System.out System System.out PrintWriter out = new PrintWriter(pr); out new PrintWriter(pr) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc double d =0.0; d 0.0 for(int i=0;i<n;i++){
            double val = sc.nextDouble();
            double ans = 1/val;
            d+=ans;
        } int i=0; int i=0; i 0 i<n i n i++ i++ i {
            double val = sc.nextDouble();
            double ans = 1/val;
            d+=ans;
        } {
            double val = sc.nextDouble();
            double ans = 1/val;
            d+=ans;
        } double val = sc.nextDouble(); val sc.nextDouble() sc.nextDouble sc double ans = 1/val; ans 1/val 1 val d+=ans d ans d=1/d d 1/d 1 d out.print(d) out.print out d out.close() out.close out String[] args args public static long gcd(long a,long b){
        if(a==0){
            return b;
        }else{
            return gcd(b%a,a);
        }
    } gcd {
        if(a==0){
            return b;
        }else{
            return gcd(b%a,a);
        }
    } if(a==0){
            return b;
        }else{
            return gcd(b%a,a);
        } a==0 a 0 {
            return b;
        } return b; b {
            return gcd(b%a,a);
        } return gcd(b%a,a); gcd(b%a,a) gcd b%a b a a long a a long b b public class Main {
	
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        OutputStream pr = System.out;
        PrintWriter out = new PrintWriter(pr);
        int n = sc.nextInt();
        double d =0.0;
        for(int i=0;i<n;i++){
            double val = sc.nextDouble();
            double ans = 1/val;
            d+=ans;
        }
        d=1/d;
        out.print(d);
        out.close();
    }
    public static long gcd(long a,long b){
        if(a==0){
            return b;
        }else{
            return gcd(b%a,a);
        }
    }
} public class Main {
	
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        OutputStream pr = System.out;
        PrintWriter out = new PrintWriter(pr);
        int n = sc.nextInt();
        double d =0.0;
        for(int i=0;i<n;i++){
            double val = sc.nextDouble();
            double ans = 1/val;
            d+=ans;
        }
        d=1/d;
        out.print(d);
        out.close();
    }
    public static long gcd(long a,long b){
        if(a==0){
            return b;
        }else{
            return gcd(b%a,a);
        }
    }
} Main
import java.util.*;
import java.io.*;
public class Main {
	
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        OutputStream pr = System.out;
        PrintWriter out = new PrintWriter(pr);
        int n = sc.nextInt();
        double d =0.0;
        for(int i=0;i<n;i++){
            double val = sc.nextDouble();
            double ans = 1/val;
            d+=ans;
        }
        d=1/d;
        out.print(d);
        out.close();
    }
    public static long gcd(long a,long b){
        if(a==0){
            return b;
        }else{
            return gcd(b%a,a);
        }
    }
}