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];
		for(int s=0;s<n;s++){
			a[s]=sc.nextInt();
		}
		double total=0;

		for(int s=0;s<n;s++){
			total+=(double)1/a[s];
		}

		
		System.out.println((double)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();
		int a[]=new int[n];
		for(int s=0;s<n;s++){
			a[s]=sc.nextInt();
		}
		double total=0;

		for(int s=0;s<n;s++){
			total+=(double)1/a[s];
		}

		
		System.out.println((double)1/total);

	}
} Main public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		int n=sc.nextInt();
		int a[]=new int[n];
		for(int s=0;s<n;s++){
			a[s]=sc.nextInt();
		}
		double total=0;

		for(int s=0;s<n;s++){
			total+=(double)1/a[s];
		}

		
		System.out.println((double)1/total);

	} main {

		Scanner sc = new Scanner(System.in);

		int n=sc.nextInt();
		int a[]=new int[n];
		for(int s=0;s<n;s++){
			a[s]=sc.nextInt();
		}
		double total=0;

		for(int s=0;s<n;s++){
			total+=(double)1/a[s];
		}

		
		System.out.println((double)1/total);

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

		for(int s=0;s<n;s++){
			total+=(double)1/a[s];
		}

		
		System.out.println((double)1/total);

	}
} public class Main {
public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		int n=sc.nextInt();
		int a[]=new int[n];
		for(int s=0;s<n;s++){
			a[s]=sc.nextInt();
		}
		double total=0;

		for(int s=0;s<n;s++){
			total+=(double)1/a[s];
		}

		
		System.out.println((double)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();
		int a[]=new int[n];
		for(int s=0;s<n;s++){
			a[s]=sc.nextInt();
		}
		double total=0;

		for(int s=0;s<n;s++){
			total+=(double)1/a[s];
		}

		
		System.out.println((double)1/total);

	}
}
