import java.util.*;

public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);

    int n = sc.nextInt();
    int[] a = new int[n];
    double numer = 1;
    double denom = 0;
    double ans;

    for(int i = 0; i < n; i++){
      a[i] = sc.nextInt();
      numer *= a[i];
    }

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

    int n = sc.nextInt();
    int[] a = new int[n];
    double numer = 1;
    double denom = 0;
    double ans;

    for(int i = 0; i < n; i++){
      a[i] = sc.nextInt();
      numer *= a[i];
    }

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

    int n = sc.nextInt();
    int[] a = new int[n];
    double numer = 1;
    double denom = 0;
    double ans;

    for(int i = 0; i < n; i++){
      a[i] = sc.nextInt();
      numer *= a[i];
    }

    for(int i = 0; i < n; i++){
      denom +=  numer / a[i];
    }
    
    ans = numer / denom;
    System.out.println(ans);
  } main {
    Scanner sc = new Scanner(System.in);

    int n = sc.nextInt();
    int[] a = new int[n];
    double numer = 1;
    double denom = 0;
    double ans;

    for(int i = 0; i < n; i++){
      a[i] = sc.nextInt();
      numer *= a[i];
    }

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

    int n = sc.nextInt();
    int[] a = new int[n];
    double numer = 1;
    double denom = 0;
    double ans;

    for(int i = 0; i < n; i++){
      a[i] = sc.nextInt();
      numer *= a[i];
    }

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

    int n = sc.nextInt();
    int[] a = new int[n];
    double numer = 1;
    double denom = 0;
    double ans;

    for(int i = 0; i < n; i++){
      a[i] = sc.nextInt();
      numer *= a[i];
    }

    for(int i = 0; i < n; i++){
      denom +=  numer / a[i];
    }
    
    ans = numer / denom;
    System.out.println(ans);
  }
} Main
import java.util.*;

public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);

    int n = sc.nextInt();
    int[] a = new int[n];
    double numer = 1;
    double denom = 0;
    double ans;

    for(int i = 0; i < n; i++){
      a[i] = sc.nextInt();
      numer *= a[i];
    }

    for(int i = 0; i < n; i++){
      denom +=  numer / a[i];
    }
    
    ans = numer / denom;
    System.out.println(ans);
  }
}
