import java.util.Scanner;
public class Main {

	public static void main(String[] args) {
		Scanner scanner=new Scanner(System.in);
		int N=scanner.nextInt();
		int[]arr=new int[N];
		for(int index=0;index<N;index++) {
			arr[index]=scanner.nextInt();
		}
		float sum=0;
		for(int value:arr) {
			sum+=1.0/value;
		}
		System.out.println(1.0*Math.round(100000/sum)/100000);
	}
} import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {
		Scanner scanner=new Scanner(System.in);
		int N=scanner.nextInt();
		int[]arr=new int[N];
		for(int index=0;index<N;index++) {
			arr[index]=scanner.nextInt();
		}
		float sum=0;
		for(int value:arr) {
			sum+=1.0/value;
		}
		System.out.println(1.0*Math.round(100000/sum)/100000);
	}
} Main public static void main(String[] args) {
		Scanner scanner=new Scanner(System.in);
		int N=scanner.nextInt();
		int[]arr=new int[N];
		for(int index=0;index<N;index++) {
			arr[index]=scanner.nextInt();
		}
		float sum=0;
		for(int value:arr) {
			sum+=1.0/value;
		}
		System.out.println(1.0*Math.round(100000/sum)/100000);
	} main {
		Scanner scanner=new Scanner(System.in);
		int N=scanner.nextInt();
		int[]arr=new int[N];
		for(int index=0;index<N;index++) {
			arr[index]=scanner.nextInt();
		}
		float sum=0;
		for(int value:arr) {
			sum+=1.0/value;
		}
		System.out.println(1.0*Math.round(100000/sum)/100000);
	} Scanner scanner=new Scanner(System.in); scanner new Scanner(System.in) int N=scanner.nextInt(); N scanner.nextInt() scanner.nextInt scanner int[]arr=new int[N]; arr new int[N] N for(int index=0;index<N;index++) {
			arr[index]=scanner.nextInt();
		} int index=0; int index=0; index 0 index<N index N index++ index++ index {
			arr[index]=scanner.nextInt();
		} {
			arr[index]=scanner.nextInt();
		} arr[index]=scanner.nextInt() arr[index] arr index scanner.nextInt() scanner.nextInt scanner float sum=0; sum 0 for(int value:arr) {
			sum+=1.0/value;
		} int value arr {
			sum+=1.0/value;
		} {
			sum+=1.0/value;
		} sum+=1.0/value sum 1.0/value 1.0 value System.out.println(1.0*Math.round(100000/sum)/100000) System.out.println System.out System System.out 1.0*Math.round(100000/sum)/100000 1.0*Math.round(100000/sum) 1.0 Math.round(100000/sum) Math.round Math 100000/sum 100000 sum 100000 String[] args args public class Main {

	public static void main(String[] args) {
		Scanner scanner=new Scanner(System.in);
		int N=scanner.nextInt();
		int[]arr=new int[N];
		for(int index=0;index<N;index++) {
			arr[index]=scanner.nextInt();
		}
		float sum=0;
		for(int value:arr) {
			sum+=1.0/value;
		}
		System.out.println(1.0*Math.round(100000/sum)/100000);
	}
} public class Main {

	public static void main(String[] args) {
		Scanner scanner=new Scanner(System.in);
		int N=scanner.nextInt();
		int[]arr=new int[N];
		for(int index=0;index<N;index++) {
			arr[index]=scanner.nextInt();
		}
		float sum=0;
		for(int value:arr) {
			sum+=1.0/value;
		}
		System.out.println(1.0*Math.round(100000/sum)/100000);
	}
} Main
import java.util.Scanner;
public class Main {

	public static void main(String[] args) {
		Scanner scanner=new Scanner(System.in);
		int N=scanner.nextInt();
		int[]arr=new int[N];
		for(int index=0;index<N;index++) {
			arr[index]=scanner.nextInt();
		}
		float sum=0;
		for(int value:arr) {
			sum+=1.0/value;
		}
		System.out.println(1.0*Math.round(100000/sum)/100000);
	}
}