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

public class Main { 

	public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	int inputScore = Integer.parseInt(sc.nextLine());
    	String[] inputNumber = sc.nextLine().split(" ");
    	double[] allNumber = new double[inputScore];
    	double all = 0;
    	for(int i = 0;i<inputScore;i++) {
    		double tmp = Double.parseDouble(inputNumber[i]);
    		all += 1/tmp;
    	}

    	
    	System.out.println(1/all);
    }
} import java.util.ArrayList; ArrayList java.util import java.util.Collections; Collections 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 sc = new Scanner(System.in);
    	int inputScore = Integer.parseInt(sc.nextLine());
    	String[] inputNumber = sc.nextLine().split(" ");
    	double[] allNumber = new double[inputScore];
    	double all = 0;
    	for(int i = 0;i<inputScore;i++) {
    		double tmp = Double.parseDouble(inputNumber[i]);
    		all += 1/tmp;
    	}

    	
    	System.out.println(1/all);
    }
} Main public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	int inputScore = Integer.parseInt(sc.nextLine());
    	String[] inputNumber = sc.nextLine().split(" ");
    	double[] allNumber = new double[inputScore];
    	double all = 0;
    	for(int i = 0;i<inputScore;i++) {
    		double tmp = Double.parseDouble(inputNumber[i]);
    		all += 1/tmp;
    	}

    	
    	System.out.println(1/all);
    } main {
    	Scanner sc = new Scanner(System.in);
    	int inputScore = Integer.parseInt(sc.nextLine());
    	String[] inputNumber = sc.nextLine().split(" ");
    	double[] allNumber = new double[inputScore];
    	double all = 0;
    	for(int i = 0;i<inputScore;i++) {
    		double tmp = Double.parseDouble(inputNumber[i]);
    		all += 1/tmp;
    	}

    	
    	System.out.println(1/all);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int inputScore = Integer.parseInt(sc.nextLine()); inputScore Integer.parseInt(sc.nextLine()) Integer.parseInt Integer sc.nextLine() sc.nextLine sc String[] inputNumber = sc.nextLine().split(" "); inputNumber sc.nextLine().split(" ") sc.nextLine().split sc.nextLine() sc.nextLine sc " " double[] allNumber = new double[inputScore]; allNumber new double[inputScore] inputScore double all = 0; all 0 for(int i = 0;i<inputScore;i++) {
    		double tmp = Double.parseDouble(inputNumber[i]);
    		all += 1/tmp;
    	} int i = 0; int i = 0; i 0 i<inputScore i inputScore i++ i++ i {
    		double tmp = Double.parseDouble(inputNumber[i]);
    		all += 1/tmp;
    	} {
    		double tmp = Double.parseDouble(inputNumber[i]);
    		all += 1/tmp;
    	} double tmp = Double.parseDouble(inputNumber[i]); tmp Double.parseDouble(inputNumber[i]) Double.parseDouble Double inputNumber[i] inputNumber i all += 1/tmp all 1/tmp 1 tmp System.out.println(1/all) System.out.println System.out System System.out 1/all 1 all String[] args args public class Main { 

	public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	int inputScore = Integer.parseInt(sc.nextLine());
    	String[] inputNumber = sc.nextLine().split(" ");
    	double[] allNumber = new double[inputScore];
    	double all = 0;
    	for(int i = 0;i<inputScore;i++) {
    		double tmp = Double.parseDouble(inputNumber[i]);
    		all += 1/tmp;
    	}

    	
    	System.out.println(1/all);
    }
} public class Main { 

	public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	int inputScore = Integer.parseInt(sc.nextLine());
    	String[] inputNumber = sc.nextLine().split(" ");
    	double[] allNumber = new double[inputScore];
    	double all = 0;
    	for(int i = 0;i<inputScore;i++) {
    		double tmp = Double.parseDouble(inputNumber[i]);
    		all += 1/tmp;
    	}

    	
    	System.out.println(1/all);
    }
} Main
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Scanner;

public class Main { 

	public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	int inputScore = Integer.parseInt(sc.nextLine());
    	String[] inputNumber = sc.nextLine().split(" ");
    	double[] allNumber = new double[inputScore];
    	double all = 0;
    	for(int i = 0;i<inputScore;i++) {
    		double tmp = Double.parseDouble(inputNumber[i]);
    		all += 1/tmp;
    	}

    	
    	System.out.println(1/all);
    }
}