import java.util.*;

class Main{
  public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    double sum = 0;
    for(int i = 0; i < n; i++){
      double a = sc.nextInt();
      sum += 1/a;
    }
    double ans = 1/sum;
    System.out.print(ans);
  }
} import java.util.*; util.* java class Main{
  public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    double sum = 0;
    for(int i = 0; i < n; i++){
      double a = sc.nextInt();
      sum += 1/a;
    }
    double ans = 1/sum;
    System.out.print(ans);
  }
} Main public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    double sum = 0;
    for(int i = 0; i < n; i++){
      double a = sc.nextInt();
      sum += 1/a;
    }
    double ans = 1/sum;
    System.out.print(ans);
  } main {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    double sum = 0;
    for(int i = 0; i < n; i++){
      double a = sc.nextInt();
      sum += 1/a;
    }
    double ans = 1/sum;
    System.out.print(ans);
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc double sum = 0; sum 0 for(int i = 0; i < n; i++){
      double a = sc.nextInt();
      sum += 1/a;
    } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
      double a = sc.nextInt();
      sum += 1/a;
    } {
      double a = sc.nextInt();
      sum += 1/a;
    } double a = sc.nextInt(); a sc.nextInt() sc.nextInt sc sum += 1/a sum 1/a 1 a double ans = 1/sum; ans 1/sum 1 sum System.out.print(ans) System.out.print System.out System System.out ans String args[] args
import java.util.*;

class Main{
  public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    double sum = 0;
    for(int i = 0; i < n; i++){
      double a = sc.nextInt();
      sum += 1/a;
    }
    double ans = 1/sum;
    System.out.print(ans);
  }
}
