import java.util.*;

class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    List<Integer>a = new ArrayList<Integer>();
    int aa;
    for(int i=0;i<n;i++){
      aa = sc.nextInt();
      a.add(aa);
    }
    double gyaku = 0;
    for(int i=0;i<n;i++){
      gyaku += 1.0/a.get(i);
    }
    System.out.println(1.0/gyaku);
  }
} import java.util.*; util.* java class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    List<Integer>a = new ArrayList<Integer>();
    int aa;
    for(int i=0;i<n;i++){
      aa = sc.nextInt();
      a.add(aa);
    }
    double gyaku = 0;
    for(int i=0;i<n;i++){
      gyaku += 1.0/a.get(i);
    }
    System.out.println(1.0/gyaku);
  }
} Main public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    List<Integer>a = new ArrayList<Integer>();
    int aa;
    for(int i=0;i<n;i++){
      aa = sc.nextInt();
      a.add(aa);
    }
    double gyaku = 0;
    for(int i=0;i<n;i++){
      gyaku += 1.0/a.get(i);
    }
    System.out.println(1.0/gyaku);
  } main {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    List<Integer>a = new ArrayList<Integer>();
    int aa;
    for(int i=0;i<n;i++){
      aa = sc.nextInt();
      a.add(aa);
    }
    double gyaku = 0;
    for(int i=0;i<n;i++){
      gyaku += 1.0/a.get(i);
    }
    System.out.println(1.0/gyaku);
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc List<Integer>a = new ArrayList<Integer>(); a new ArrayList<Integer>() int aa; aa for(int i=0;i<n;i++){
      aa = sc.nextInt();
      a.add(aa);
    } int i=0; int i=0; i 0 i<n i n i++ i++ i {
      aa = sc.nextInt();
      a.add(aa);
    } {
      aa = sc.nextInt();
      a.add(aa);
    } aa = sc.nextInt() aa sc.nextInt() sc.nextInt sc a.add(aa) a.add a aa double gyaku = 0; gyaku 0 for(int i=0;i<n;i++){
      gyaku += 1.0/a.get(i);
    } int i=0; int i=0; i 0 i<n i n i++ i++ i {
      gyaku += 1.0/a.get(i);
    } {
      gyaku += 1.0/a.get(i);
    } gyaku += 1.0/a.get(i) gyaku 1.0/a.get(i) 1.0 a.get(i) a.get a i System.out.println(1.0/gyaku) System.out.println System.out System System.out 1.0/gyaku 1.0 gyaku String[] args args
import java.util.*;

class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    List<Integer>a = new ArrayList<Integer>();
    int aa;
    for(int i=0;i<n;i++){
      aa = sc.nextInt();
      a.add(aa);
    }
    double gyaku = 0;
    for(int i=0;i<n;i++){
      gyaku += 1.0/a.get(i);
    }
    System.out.println(1.0/gyaku);
  }
}
