import java.util.*;//ABC138-B,再度復習
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 x = 0;
    double y = 0;

    for(int i=0 ; i<n ; i++){
      a[i] = sc.nextInt();
    }
    for(int i=0 ; i<n ; i++){
      x += 1/(double)a[i];
    }
    y = 1/(double)x;
    System.out.println(y);
  }
} import java.util.*;//ABC138-B,再度復習 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 x = 0;
    double y = 0;

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

    for(int i=0 ; i<n ; i++){
      a[i] = sc.nextInt();
    }
    for(int i=0 ; i<n ; i++){
      x += 1/(double)a[i];
    }
    y = 1/(double)x;
    System.out.println(y);
  } main {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int a[] = new int[n];
    double x = 0;
    double y = 0;

    for(int i=0 ; i<n ; i++){
      a[i] = sc.nextInt();
    }
    for(int i=0 ; i<n ; i++){
      x += 1/(double)a[i];
    }
    y = 1/(double)x;
    System.out.println(y);
  } 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 x = 0; x 0 double y = 0; y 0 for(int i=0 ; i<n ; i++){
      a[i] = sc.nextInt();
    } int i=0 ; int i=0 ; i 0 i<n i n i++ i++ i {
      a[i] = sc.nextInt();
    } {
      a[i] = sc.nextInt();
    } a[i] = sc.nextInt() a[i] a i sc.nextInt() sc.nextInt sc for(int i=0 ; i<n ; i++){
      x += 1/(double)a[i];
    } int i=0 ; int i=0 ; i 0 i<n i n i++ i++ i {
      x += 1/(double)a[i];
    } {
      x += 1/(double)a[i];
    } x += 1/(double)a[i] x 1/(double)a[i] 1 (double)a[i] a i y = 1/(double)x y 1/(double)x 1 (double)x System.out.println(y) System.out.println System.out System System.out y 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 x = 0;
    double y = 0;

    for(int i=0 ; i<n ; i++){
      a[i] = sc.nextInt();
    }
    for(int i=0 ; i<n ; i++){
      x += 1/(double)a[i];
    }
    y = 1/(double)x;
    System.out.println(y);
  }
} 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 x = 0;
    double y = 0;

    for(int i=0 ; i<n ; i++){
      a[i] = sc.nextInt();
    }
    for(int i=0 ; i<n ; i++){
      x += 1/(double)a[i];
    }
    y = 1/(double)x;
    System.out.println(y);
  }
} Main
import java.util.*;//ABC138-B,再度復習
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 x = 0;
    double y = 0;

    for(int i=0 ; i<n ; i++){
      a[i] = sc.nextInt();
    }
    for(int i=0 ; i<n ; i++){
      x += 1/(double)a[i];
    }
    y = 1/(double)x;
    System.out.println(y);
  }
}
