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