import java.util.*;

class Main{
	public static void main(String[] $){
		//入力
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] a = new int[n];
		for(int i = 0; i < n; i++) a[i] = sc.nextInt();
		sc.close();

		//処理
		double ans = -1;
		double sum = 0;
		for(int i = 0; i < n; i++){
			sum += (double)1/a[i];
		}
		ans = (double)1 / sum;

		//出力
		System.out.println(ans);
	}
} import java.util.*; util.* java class Main{
	public static void main(String[] $){
		//入力
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] a = new int[n];
		for(int i = 0; i < n; i++) a[i] = sc.nextInt();
		sc.close();

		//処理
		double ans = -1;
		double sum = 0;
		for(int i = 0; i < n; i++){
			sum += (double)1/a[i];
		}
		ans = (double)1 / sum;

		//出力
		System.out.println(ans);
	}
} Main public static void main(String[] $){
		//入力
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] a = new int[n];
		for(int i = 0; i < n; i++) a[i] = sc.nextInt();
		sc.close();

		//処理
		double ans = -1;
		double sum = 0;
		for(int i = 0; i < n; i++){
			sum += (double)1/a[i];
		}
		ans = (double)1 / sum;

		//出力
		System.out.println(ans);
	} main {
		//入力
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] a = new int[n];
		for(int i = 0; i < n; i++) a[i] = sc.nextInt();
		sc.close();

		//処理
		double ans = -1;
		double sum = 0;
		for(int i = 0; i < n; i++){
			sum += (double)1/a[i];
		}
		ans = (double)1 / sum;

		//出力
		System.out.println(ans);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int[] a = new int[n]; a new int[n] n for(int i = 0; i < n; i++) a[i] = sc.nextInt(); int i = 0; int i = 0; i 0 i < n i n i++ i++ i a[i] = sc.nextInt(); a[i] = sc.nextInt() a[i] a i sc.nextInt() sc.nextInt sc sc.close() sc.close sc double ans = -1; ans -1 1 double sum = 0; sum 0 for(int i = 0; i < n; i++){
			sum += (double)1/a[i];
		} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
			sum += (double)1/a[i];
		} {
			sum += (double)1/a[i];
		} sum += (double)1/a[i] sum (double)1/a[i] (double)1 a[i] a i ans = (double)1 / sum ans (double)1 / sum (double)1 sum System.out.println(ans) System.out.println System.out System System.out ans String[] $ $
import java.util.*;

class Main{
	public static void main(String[] $){
		//入力
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] a = new int[n];
		for(int i = 0; i < n; i++) a[i] = sc.nextInt();
		sc.close();

		//処理
		double ans = -1;
		double sum = 0;
		for(int i = 0; i < n; i++){
			sum += (double)1/a[i];
		}
		ans = (double)1 / sum;

		//出力
		System.out.println(ans);
	}
}
