import java.util.*;
class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    double ans = 0.0;
    double a;

    for(int i=0;i<n;i++) {
      a=sc.nextDouble();
      ans = ans+ 1/a;
    }
    ans = 1/ans;
    System.out.println(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 ans = 0.0;
    double a;

    for(int i=0;i<n;i++) {
      a=sc.nextDouble();
      ans = ans+ 1/a;
    }
    ans = 1/ans;
    System.out.println(ans);
    
}
} Main public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    double ans = 0.0;
    double a;

    for(int i=0;i<n;i++) {
      a=sc.nextDouble();
      ans = ans+ 1/a;
    }
    ans = 1/ans;
    System.out.println(ans);
    
} main {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    double ans = 0.0;
    double a;

    for(int i=0;i<n;i++) {
      a=sc.nextDouble();
      ans = ans+ 1/a;
    }
    ans = 1/ans;
    System.out.println(ans);
    
} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc double ans = 0.0; ans 0.0 double a; a for(int i=0;i<n;i++) {
      a=sc.nextDouble();
      ans = ans+ 1/a;
    } int i=0; int i=0; i 0 i<n i n i++ i++ i {
      a=sc.nextDouble();
      ans = ans+ 1/a;
    } {
      a=sc.nextDouble();
      ans = ans+ 1/a;
    } a=sc.nextDouble() a sc.nextDouble() sc.nextDouble sc ans = ans+ 1/a ans ans+ 1/a ans 1/a 1 a ans = 1/ans ans 1/ans 1 ans 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 sc = new Scanner(System.in);
    int n = sc.nextInt();
    double ans = 0.0;
    double a;

    for(int i=0;i<n;i++) {
      a=sc.nextDouble();
      ans = ans+ 1/a;
    }
    ans = 1/ans;
    System.out.println(ans);
    
}
}
