import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Main {
	public static void main(String[] args){
		Scanner scan = new Scanner(System.in);
		int N =scan.nextInt();
		Double a;
		double c=0;
		List<Double> b=new ArrayList<Double>();
		for(int count=0;count<N;count++){
			a=(double) scan.nextInt();
			b.add(1/a);
		}
		scan.close();
		for(int count=0;count<b.size();count++){
			c=b.get(count)+c;
		}
		System.out.println(1/c);
	}
} import java.util.ArrayList; ArrayList java.util import java.util.List; List java.util import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args){
		Scanner scan = new Scanner(System.in);
		int N =scan.nextInt();
		Double a;
		double c=0;
		List<Double> b=new ArrayList<Double>();
		for(int count=0;count<N;count++){
			a=(double) scan.nextInt();
			b.add(1/a);
		}
		scan.close();
		for(int count=0;count<b.size();count++){
			c=b.get(count)+c;
		}
		System.out.println(1/c);
	}
} Main public static void main(String[] args){
		Scanner scan = new Scanner(System.in);
		int N =scan.nextInt();
		Double a;
		double c=0;
		List<Double> b=new ArrayList<Double>();
		for(int count=0;count<N;count++){
			a=(double) scan.nextInt();
			b.add(1/a);
		}
		scan.close();
		for(int count=0;count<b.size();count++){
			c=b.get(count)+c;
		}
		System.out.println(1/c);
	} main {
		Scanner scan = new Scanner(System.in);
		int N =scan.nextInt();
		Double a;
		double c=0;
		List<Double> b=new ArrayList<Double>();
		for(int count=0;count<N;count++){
			a=(double) scan.nextInt();
			b.add(1/a);
		}
		scan.close();
		for(int count=0;count<b.size();count++){
			c=b.get(count)+c;
		}
		System.out.println(1/c);
	} Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int N =scan.nextInt(); N scan.nextInt() scan.nextInt scan Double a; a double c=0; c 0 List<Double> b=new ArrayList<Double>(); b new ArrayList<Double>() for(int count=0;count<N;count++){
			a=(double) scan.nextInt();
			b.add(1/a);
		} int count=0; int count=0; count 0 count<N count N count++ count++ count {
			a=(double) scan.nextInt();
			b.add(1/a);
		} {
			a=(double) scan.nextInt();
			b.add(1/a);
		} a=(double) scan.nextInt() a (double) scan.nextInt() scan.nextInt scan b.add(1/a) b.add b 1/a 1 a scan.close() scan.close scan for(int count=0;count<b.size();count++){
			c=b.get(count)+c;
		} int count=0; int count=0; count 0 count<b.size() count b.size() b.size b count++ count++ count {
			c=b.get(count)+c;
		} {
			c=b.get(count)+c;
		} c=b.get(count)+c c b.get(count)+c b.get(count) b.get b count c System.out.println(1/c) System.out.println System.out System System.out 1/c 1 c String[] args args public class Main {
	public static void main(String[] args){
		Scanner scan = new Scanner(System.in);
		int N =scan.nextInt();
		Double a;
		double c=0;
		List<Double> b=new ArrayList<Double>();
		for(int count=0;count<N;count++){
			a=(double) scan.nextInt();
			b.add(1/a);
		}
		scan.close();
		for(int count=0;count<b.size();count++){
			c=b.get(count)+c;
		}
		System.out.println(1/c);
	}
} public class Main {
	public static void main(String[] args){
		Scanner scan = new Scanner(System.in);
		int N =scan.nextInt();
		Double a;
		double c=0;
		List<Double> b=new ArrayList<Double>();
		for(int count=0;count<N;count++){
			a=(double) scan.nextInt();
			b.add(1/a);
		}
		scan.close();
		for(int count=0;count<b.size();count++){
			c=b.get(count)+c;
		}
		System.out.println(1/c);
	}
} Main
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Main {
	public static void main(String[] args){
		Scanner scan = new Scanner(System.in);
		int N =scan.nextInt();
		Double a;
		double c=0;
		List<Double> b=new ArrayList<Double>();
		for(int count=0;count<N;count++){
			a=(double) scan.nextInt();
			b.add(1/a);
		}
		scan.close();
		for(int count=0;count<b.size();count++){
			c=b.get(count)+c;
		}
		System.out.println(1/c);
	}
}
