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

class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int N = scan.nextInt();
    double S = 0 ;
    double[] A = new double[N];
    for(int i = 0;i<N;i++){
      A[i] = scan.nextDouble();
      A[i] =1/A[i];
      S+=A[i];
    }
    System.out.println(1/S);
  }
} import java.io.*; io.* java import java.util.*; util.* java import java.lang.*; lang.* java class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int N = scan.nextInt();
    double S = 0 ;
    double[] A = new double[N];
    for(int i = 0;i<N;i++){
      A[i] = scan.nextDouble();
      A[i] =1/A[i];
      S+=A[i];
    }
    System.out.println(1/S);
  }
} Main public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int N = scan.nextInt();
    double S = 0 ;
    double[] A = new double[N];
    for(int i = 0;i<N;i++){
      A[i] = scan.nextDouble();
      A[i] =1/A[i];
      S+=A[i];
    }
    System.out.println(1/S);
  } main {
    Scanner scan = new Scanner(System.in);
    int N = scan.nextInt();
    double S = 0 ;
    double[] A = new double[N];
    for(int i = 0;i<N;i++){
      A[i] = scan.nextDouble();
      A[i] =1/A[i];
      S+=A[i];
    }
    System.out.println(1/S);
  } Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int N = scan.nextInt(); N scan.nextInt() scan.nextInt scan double S = 0 ; S 0 double[] A = new double[N]; A new double[N] N for(int i = 0;i<N;i++){
      A[i] = scan.nextDouble();
      A[i] =1/A[i];
      S+=A[i];
    } int i = 0; int i = 0; i 0 i<N i N i++ i++ i {
      A[i] = scan.nextDouble();
      A[i] =1/A[i];
      S+=A[i];
    } {
      A[i] = scan.nextDouble();
      A[i] =1/A[i];
      S+=A[i];
    } A[i] = scan.nextDouble() A[i] A i scan.nextDouble() scan.nextDouble scan A[i] =1/A[i] A[i] A i 1/A[i] 1 A[i] A i S+=A[i] S A[i] A i System.out.println(1/S) System.out.println System.out System System.out 1/S 1 S String[] args args
import java.io.*;
import java.util.*;
import java.lang.*;

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