import java.util.*;
public class Main{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int arr[] = new int[n];
		double d =0;
		for(int i=0;i<n;i++){
			arr[i]=sc.nextInt();
			d+=(1.0/arr[i]);
		}	
		System.out.println(1.0/d);
	}
} 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 arr[] = new int[n];
		double d =0;
		for(int i=0;i<n;i++){
			arr[i]=sc.nextInt();
			d+=(1.0/arr[i]);
		}	
		System.out.println(1.0/d);
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int arr[] = new int[n];
		double d =0;
		for(int i=0;i<n;i++){
			arr[i]=sc.nextInt();
			d+=(1.0/arr[i]);
		}	
		System.out.println(1.0/d);
	} main {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int arr[] = new int[n];
		double d =0;
		for(int i=0;i<n;i++){
			arr[i]=sc.nextInt();
			d+=(1.0/arr[i]);
		}	
		System.out.println(1.0/d);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int arr[] = new int[n]; arr new int[n] n double d =0; d 0 for(int i=0;i<n;i++){
			arr[i]=sc.nextInt();
			d+=(1.0/arr[i]);
		} int i=0; int i=0; i 0 i<n i n i++ i++ i {
			arr[i]=sc.nextInt();
			d+=(1.0/arr[i]);
		} {
			arr[i]=sc.nextInt();
			d+=(1.0/arr[i]);
		} arr[i]=sc.nextInt() arr[i] arr i sc.nextInt() sc.nextInt sc d+=(1.0/arr[i]) d (1.0/arr[i]) 1.0 arr[i] arr i System.out.println(1.0/d) System.out.println System.out System System.out 1.0/d 1.0 d String[] args args public class Main{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int arr[] = new int[n];
		double d =0;
		for(int i=0;i<n;i++){
			arr[i]=sc.nextInt();
			d+=(1.0/arr[i]);
		}	
		System.out.println(1.0/d);
	}
} public class Main{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int arr[] = new int[n];
		double d =0;
		for(int i=0;i<n;i++){
			arr[i]=sc.nextInt();
			d+=(1.0/arr[i]);
		}	
		System.out.println(1.0/d);
	}
} Main
import java.util.*;
public class Main{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int arr[] = new int[n];
		double d =0;
		for(int i=0;i<n;i++){
			arr[i]=sc.nextInt();
			d+=(1.0/arr[i]);
		}	
		System.out.println(1.0/d);
	}
}