import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scn = new Scanner(System.in);
		int n = scn.nextInt();
		double m = 0;
		for(int i = 0 ;i < n;i++) {
			 m+=1/scn.nextDouble();
		}
		System.out.println(1/m);
		scn.close();
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) {
		Scanner scn = new Scanner(System.in);
		int n = scn.nextInt();
		double m = 0;
		for(int i = 0 ;i < n;i++) {
			 m+=1/scn.nextDouble();
		}
		System.out.println(1/m);
		scn.close();
	}
} Main public static void main(String[] args) {
		Scanner scn = new Scanner(System.in);
		int n = scn.nextInt();
		double m = 0;
		for(int i = 0 ;i < n;i++) {
			 m+=1/scn.nextDouble();
		}
		System.out.println(1/m);
		scn.close();
	} main {
		Scanner scn = new Scanner(System.in);
		int n = scn.nextInt();
		double m = 0;
		for(int i = 0 ;i < n;i++) {
			 m+=1/scn.nextDouble();
		}
		System.out.println(1/m);
		scn.close();
	} Scanner scn = new Scanner(System.in); scn new Scanner(System.in) int n = scn.nextInt(); n scn.nextInt() scn.nextInt scn double m = 0; m 0 for(int i = 0 ;i < n;i++) {
			 m+=1/scn.nextDouble();
		} int i = 0 ; int i = 0 ; i 0 i < n i n i++ i++ i {
			 m+=1/scn.nextDouble();
		} {
			 m+=1/scn.nextDouble();
		} m+=1/scn.nextDouble() m 1/scn.nextDouble() 1 scn.nextDouble() scn.nextDouble scn System.out.println(1/m) System.out.println System.out System System.out 1/m 1 m scn.close() scn.close scn String[] args args public class Main {
	public static void main(String[] args) {
		Scanner scn = new Scanner(System.in);
		int n = scn.nextInt();
		double m = 0;
		for(int i = 0 ;i < n;i++) {
			 m+=1/scn.nextDouble();
		}
		System.out.println(1/m);
		scn.close();
	}
} public class Main {
	public static void main(String[] args) {
		Scanner scn = new Scanner(System.in);
		int n = scn.nextInt();
		double m = 0;
		for(int i = 0 ;i < n;i++) {
			 m+=1/scn.nextDouble();
		}
		System.out.println(1/m);
		scn.close();
	}
} Main
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scn = new Scanner(System.in);
		int n = scn.nextInt();
		double m = 0;
		for(int i = 0 ;i < n;i++) {
			 m+=1/scn.nextDouble();
		}
		System.out.println(1/m);
		scn.close();
	}
}