import java.util.*;

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

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