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

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int totalNum = sc.nextInt();
		double[] list = new double[totalNum];
		for(int i = 0; i < totalNum; i++) {
			list[i] = (double)sc.nextInt();
		}
		double reciprocalSum = 0;
		for(int i = 0; i < totalNum; i++) {
			reciprocalSum += 1 / list[i];
		}
		double ans = 1 / reciprocalSum;
		System.out.println(ans);
	}
} import java.math.BigDecimal; BigDecimal java.math import java.util.Scanner; Scanner java.util import java.math.RoundingMode; RoundingMode java.math public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int totalNum = sc.nextInt();
		double[] list = new double[totalNum];
		for(int i = 0; i < totalNum; i++) {
			list[i] = (double)sc.nextInt();
		}
		double reciprocalSum = 0;
		for(int i = 0; i < totalNum; i++) {
			reciprocalSum += 1 / list[i];
		}
		double ans = 1 / reciprocalSum;
		System.out.println(ans);
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int totalNum = sc.nextInt();
		double[] list = new double[totalNum];
		for(int i = 0; i < totalNum; i++) {
			list[i] = (double)sc.nextInt();
		}
		double reciprocalSum = 0;
		for(int i = 0; i < totalNum; i++) {
			reciprocalSum += 1 / list[i];
		}
		double ans = 1 / reciprocalSum;
		System.out.println(ans);
	} main {
		Scanner sc = new Scanner(System.in);
		int totalNum = sc.nextInt();
		double[] list = new double[totalNum];
		for(int i = 0; i < totalNum; i++) {
			list[i] = (double)sc.nextInt();
		}
		double reciprocalSum = 0;
		for(int i = 0; i < totalNum; i++) {
			reciprocalSum += 1 / list[i];
		}
		double ans = 1 / reciprocalSum;
		System.out.println(ans);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int totalNum = sc.nextInt(); totalNum sc.nextInt() sc.nextInt sc double[] list = new double[totalNum]; list new double[totalNum] totalNum for(int i = 0; i < totalNum; i++) {
			list[i] = (double)sc.nextInt();
		} int i = 0; int i = 0; i 0 i < totalNum i totalNum i++ i++ i {
			list[i] = (double)sc.nextInt();
		} {
			list[i] = (double)sc.nextInt();
		} list[i] = (double)sc.nextInt() list[i] list i (double)sc.nextInt() sc.nextInt sc double reciprocalSum = 0; reciprocalSum 0 for(int i = 0; i < totalNum; i++) {
			reciprocalSum += 1 / list[i];
		} int i = 0; int i = 0; i 0 i < totalNum i totalNum i++ i++ i {
			reciprocalSum += 1 / list[i];
		} {
			reciprocalSum += 1 / list[i];
		} reciprocalSum += 1 / list[i] reciprocalSum 1 / list[i] 1 list[i] list i double ans = 1 / reciprocalSum; ans 1 / reciprocalSum 1 reciprocalSum System.out.println(ans) System.out.println System.out System System.out ans String[] args args public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int totalNum = sc.nextInt();
		double[] list = new double[totalNum];
		for(int i = 0; i < totalNum; i++) {
			list[i] = (double)sc.nextInt();
		}
		double reciprocalSum = 0;
		for(int i = 0; i < totalNum; i++) {
			reciprocalSum += 1 / list[i];
		}
		double ans = 1 / reciprocalSum;
		System.out.println(ans);
	}
} public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int totalNum = sc.nextInt();
		double[] list = new double[totalNum];
		for(int i = 0; i < totalNum; i++) {
			list[i] = (double)sc.nextInt();
		}
		double reciprocalSum = 0;
		for(int i = 0; i < totalNum; i++) {
			reciprocalSum += 1 / list[i];
		}
		double ans = 1 / reciprocalSum;
		System.out.println(ans);
	}
} Main
import java.math.BigDecimal;
import java.util.Scanner;
import java.math.RoundingMode;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int totalNum = sc.nextInt();
		double[] list = new double[totalNum];
		for(int i = 0; i < totalNum; i++) {
			list[i] = (double)sc.nextInt();
		}
		double reciprocalSum = 0;
		for(int i = 0; i < totalNum; i++) {
			reciprocalSum += 1 / list[i];
		}
		double ans = 1 / reciprocalSum;
		System.out.println(ans);
	}
}