import java.math.BigDecimal;
import java.util.Scanner;

public class Main {
    public static void main(String args[] ) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        BigDecimal sum = new BigDecimal(0);
        for (int i = 0; i < n; i++) {
        	int num = sc.nextInt();
        	new BigDecimal(num);
        	BigDecimal gyaku =  new BigDecimal(1).divide(new BigDecimal(num), 10, BigDecimal.ROUND_DOWN);
        	sum = sum.add(gyaku);
		}
        System.out.println(new BigDecimal(1).divide(sum, 10, BigDecimal.ROUND_DOWN).toString());
    }
} import java.math.BigDecimal; BigDecimal java.math import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String args[] ) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        BigDecimal sum = new BigDecimal(0);
        for (int i = 0; i < n; i++) {
        	int num = sc.nextInt();
        	new BigDecimal(num);
        	BigDecimal gyaku =  new BigDecimal(1).divide(new BigDecimal(num), 10, BigDecimal.ROUND_DOWN);
        	sum = sum.add(gyaku);
		}
        System.out.println(new BigDecimal(1).divide(sum, 10, BigDecimal.ROUND_DOWN).toString());
    }
} Main public static void main(String args[] ) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        BigDecimal sum = new BigDecimal(0);
        for (int i = 0; i < n; i++) {
        	int num = sc.nextInt();
        	new BigDecimal(num);
        	BigDecimal gyaku =  new BigDecimal(1).divide(new BigDecimal(num), 10, BigDecimal.ROUND_DOWN);
        	sum = sum.add(gyaku);
		}
        System.out.println(new BigDecimal(1).divide(sum, 10, BigDecimal.ROUND_DOWN).toString());
    } main {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        BigDecimal sum = new BigDecimal(0);
        for (int i = 0; i < n; i++) {
        	int num = sc.nextInt();
        	new BigDecimal(num);
        	BigDecimal gyaku =  new BigDecimal(1).divide(new BigDecimal(num), 10, BigDecimal.ROUND_DOWN);
        	sum = sum.add(gyaku);
		}
        System.out.println(new BigDecimal(1).divide(sum, 10, BigDecimal.ROUND_DOWN).toString());
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc BigDecimal sum = new BigDecimal(0); sum new BigDecimal(0) for (int i = 0; i < n; i++) {
        	int num = sc.nextInt();
        	new BigDecimal(num);
        	BigDecimal gyaku =  new BigDecimal(1).divide(new BigDecimal(num), 10, BigDecimal.ROUND_DOWN);
        	sum = sum.add(gyaku);
		} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
        	int num = sc.nextInt();
        	new BigDecimal(num);
        	BigDecimal gyaku =  new BigDecimal(1).divide(new BigDecimal(num), 10, BigDecimal.ROUND_DOWN);
        	sum = sum.add(gyaku);
		} {
        	int num = sc.nextInt();
        	new BigDecimal(num);
        	BigDecimal gyaku =  new BigDecimal(1).divide(new BigDecimal(num), 10, BigDecimal.ROUND_DOWN);
        	sum = sum.add(gyaku);
		} int num = sc.nextInt(); num sc.nextInt() sc.nextInt sc new BigDecimal(num) BigDecimal gyaku =  new BigDecimal(1).divide(new BigDecimal(num), 10, BigDecimal.ROUND_DOWN); gyaku new BigDecimal(1).divide(new BigDecimal(num), 10, BigDecimal.ROUND_DOWN) new BigDecimal(1).divide new BigDecimal(1) new BigDecimal(num) 10 BigDecimal.ROUND_DOWN BigDecimal BigDecimal.ROUND_DOWN sum = sum.add(gyaku) sum sum.add(gyaku) sum.add sum gyaku System.out.println(new BigDecimal(1).divide(sum, 10, BigDecimal.ROUND_DOWN).toString()) System.out.println System.out System System.out new BigDecimal(1).divide(sum, 10, BigDecimal.ROUND_DOWN).toString() new BigDecimal(1).divide(sum, 10, BigDecimal.ROUND_DOWN).toString new BigDecimal(1).divide(sum, 10, BigDecimal.ROUND_DOWN) new BigDecimal(1).divide new BigDecimal(1) sum 10 BigDecimal.ROUND_DOWN BigDecimal BigDecimal.ROUND_DOWN String args[] args public class Main {
    public static void main(String args[] ) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        BigDecimal sum = new BigDecimal(0);
        for (int i = 0; i < n; i++) {
        	int num = sc.nextInt();
        	new BigDecimal(num);
        	BigDecimal gyaku =  new BigDecimal(1).divide(new BigDecimal(num), 10, BigDecimal.ROUND_DOWN);
        	sum = sum.add(gyaku);
		}
        System.out.println(new BigDecimal(1).divide(sum, 10, BigDecimal.ROUND_DOWN).toString());
    }
} public class Main {
    public static void main(String args[] ) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        BigDecimal sum = new BigDecimal(0);
        for (int i = 0; i < n; i++) {
        	int num = sc.nextInt();
        	new BigDecimal(num);
        	BigDecimal gyaku =  new BigDecimal(1).divide(new BigDecimal(num), 10, BigDecimal.ROUND_DOWN);
        	sum = sum.add(gyaku);
		}
        System.out.println(new BigDecimal(1).divide(sum, 10, BigDecimal.ROUND_DOWN).toString());
    }
} Main
import java.math.BigDecimal;
import java.util.Scanner;

public class Main {
    public static void main(String args[] ) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        BigDecimal sum = new BigDecimal(0);
        for (int i = 0; i < n; i++) {
        	int num = sc.nextInt();
        	new BigDecimal(num);
        	BigDecimal gyaku =  new BigDecimal(1).divide(new BigDecimal(num), 10, BigDecimal.ROUND_DOWN);
        	sum = sum.add(gyaku);
		}
        System.out.println(new BigDecimal(1).divide(sum, 10, BigDecimal.ROUND_DOWN).toString());
    }
}
