import java.io.PrintWriter;
import java.util.*;

public class Main {
	public static void main(String[] args) {
		Scanner stdin = new Scanner(System.in);
		PrintWriter out = new PrintWriter(System.out);
		
		int N = Integer.parseInt(stdin.next());
		double A[] = new double[N];
		double sum=0;
		
		for(int i=0;i<N;i++) {
			A[i] = Integer.parseInt(stdin.next());
		}
		
		for(int i=0;i<N;i++) {
			sum += 1/A[i];
		}
		
		out.println(1/sum);
		
		out.flush();
	}
	
} import java.io.PrintWriter; PrintWriter java.io import java.util.*; util.* java public class Main {
	public static void main(String[] args) {
		Scanner stdin = new Scanner(System.in);
		PrintWriter out = new PrintWriter(System.out);
		
		int N = Integer.parseInt(stdin.next());
		double A[] = new double[N];
		double sum=0;
		
		for(int i=0;i<N;i++) {
			A[i] = Integer.parseInt(stdin.next());
		}
		
		for(int i=0;i<N;i++) {
			sum += 1/A[i];
		}
		
		out.println(1/sum);
		
		out.flush();
	}
	
} Main public static void main(String[] args) {
		Scanner stdin = new Scanner(System.in);
		PrintWriter out = new PrintWriter(System.out);
		
		int N = Integer.parseInt(stdin.next());
		double A[] = new double[N];
		double sum=0;
		
		for(int i=0;i<N;i++) {
			A[i] = Integer.parseInt(stdin.next());
		}
		
		for(int i=0;i<N;i++) {
			sum += 1/A[i];
		}
		
		out.println(1/sum);
		
		out.flush();
	} main {
		Scanner stdin = new Scanner(System.in);
		PrintWriter out = new PrintWriter(System.out);
		
		int N = Integer.parseInt(stdin.next());
		double A[] = new double[N];
		double sum=0;
		
		for(int i=0;i<N;i++) {
			A[i] = Integer.parseInt(stdin.next());
		}
		
		for(int i=0;i<N;i++) {
			sum += 1/A[i];
		}
		
		out.println(1/sum);
		
		out.flush();
	} Scanner stdin = new Scanner(System.in); stdin new Scanner(System.in) PrintWriter out = new PrintWriter(System.out); out new PrintWriter(System.out) int N = Integer.parseInt(stdin.next()); N Integer.parseInt(stdin.next()) Integer.parseInt Integer stdin.next() stdin.next stdin double A[] = new double[N]; A new double[N] N double sum=0; sum 0 for(int i=0;i<N;i++) {
			A[i] = Integer.parseInt(stdin.next());
		} int i=0; int i=0; i 0 i<N i N i++ i++ i {
			A[i] = Integer.parseInt(stdin.next());
		} {
			A[i] = Integer.parseInt(stdin.next());
		} A[i] = Integer.parseInt(stdin.next()) A[i] A i Integer.parseInt(stdin.next()) Integer.parseInt Integer stdin.next() stdin.next stdin for(int i=0;i<N;i++) {
			sum += 1/A[i];
		} int i=0; int i=0; i 0 i<N i N i++ i++ i {
			sum += 1/A[i];
		} {
			sum += 1/A[i];
		} sum += 1/A[i] sum 1/A[i] 1 A[i] A i out.println(1/sum) out.println out 1/sum 1 sum out.flush() out.flush out String[] args args public class Main {
	public static void main(String[] args) {
		Scanner stdin = new Scanner(System.in);
		PrintWriter out = new PrintWriter(System.out);
		
		int N = Integer.parseInt(stdin.next());
		double A[] = new double[N];
		double sum=0;
		
		for(int i=0;i<N;i++) {
			A[i] = Integer.parseInt(stdin.next());
		}
		
		for(int i=0;i<N;i++) {
			sum += 1/A[i];
		}
		
		out.println(1/sum);
		
		out.flush();
	}
	
} public class Main {
	public static void main(String[] args) {
		Scanner stdin = new Scanner(System.in);
		PrintWriter out = new PrintWriter(System.out);
		
		int N = Integer.parseInt(stdin.next());
		double A[] = new double[N];
		double sum=0;
		
		for(int i=0;i<N;i++) {
			A[i] = Integer.parseInt(stdin.next());
		}
		
		for(int i=0;i<N;i++) {
			sum += 1/A[i];
		}
		
		out.println(1/sum);
		
		out.flush();
	}
	
} Main
import java.io.PrintWriter;
import java.util.*;

public class Main {
	public static void main(String[] args) {
		Scanner stdin = new Scanner(System.in);
		PrintWriter out = new PrintWriter(System.out);
		
		int N = Integer.parseInt(stdin.next());
		double A[] = new double[N];
		double sum=0;
		
		for(int i=0;i<N;i++) {
			A[i] = Integer.parseInt(stdin.next());
		}
		
		for(int i=0;i<N;i++) {
			sum += 1/A[i];
		}
		
		out.println(1/sum);
		
		out.flush();
	}
	
}