import java.util.*;
class Main{
  public static void main(String[] args){
	Scanner stdIn = new Scanner(System.in);
	int N = stdIn.nextInt();
    double[] a=new double[N];
    double sum=0.0;
    for(int i=0;i<N;i++){
      a[i]=Double.parseDouble(stdIn.next());
      sum+=1.0/a[i];
    }
    System.out.println(1.0/sum);
  }
} import java.util.*; util.* java class Main{
  public static void main(String[] args){
	Scanner stdIn = new Scanner(System.in);
	int N = stdIn.nextInt();
    double[] a=new double[N];
    double sum=0.0;
    for(int i=0;i<N;i++){
      a[i]=Double.parseDouble(stdIn.next());
      sum+=1.0/a[i];
    }
    System.out.println(1.0/sum);
  }
} Main public static void main(String[] args){
	Scanner stdIn = new Scanner(System.in);
	int N = stdIn.nextInt();
    double[] a=new double[N];
    double sum=0.0;
    for(int i=0;i<N;i++){
      a[i]=Double.parseDouble(stdIn.next());
      sum+=1.0/a[i];
    }
    System.out.println(1.0/sum);
  } main {
	Scanner stdIn = new Scanner(System.in);
	int N = stdIn.nextInt();
    double[] a=new double[N];
    double sum=0.0;
    for(int i=0;i<N;i++){
      a[i]=Double.parseDouble(stdIn.next());
      sum+=1.0/a[i];
    }
    System.out.println(1.0/sum);
  } Scanner stdIn = new Scanner(System.in); stdIn new Scanner(System.in) int N = stdIn.nextInt(); N stdIn.nextInt() stdIn.nextInt stdIn double[] a=new double[N]; a new double[N] N double sum=0.0; sum 0.0 for(int i=0;i<N;i++){
      a[i]=Double.parseDouble(stdIn.next());
      sum+=1.0/a[i];
    } int i=0; int i=0; i 0 i<N i N i++ i++ i {
      a[i]=Double.parseDouble(stdIn.next());
      sum+=1.0/a[i];
    } {
      a[i]=Double.parseDouble(stdIn.next());
      sum+=1.0/a[i];
    } a[i]=Double.parseDouble(stdIn.next()) a[i] a i Double.parseDouble(stdIn.next()) Double.parseDouble Double stdIn.next() stdIn.next stdIn sum+=1.0/a[i] sum 1.0/a[i] 1.0 a[i] a i System.out.println(1.0/sum) System.out.println System.out System System.out 1.0/sum 1.0 sum String[] args args
import java.util.*;
class Main{
  public static void main(String[] args){
	Scanner stdIn = new Scanner(System.in);
	int N = stdIn.nextInt();
    double[] a=new double[N];
    double sum=0.0;
    for(int i=0;i<N;i++){
      a[i]=Double.parseDouble(stdIn.next());
      sum+=1.0/a[i];
    }
    System.out.println(1.0/sum);
  }
}