import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int N = sc.nextInt();
      
	    double[] A = new double[N];
	    double[] gA = new double[N];
	    double num = 0;
    
  	    for(int i = 0 ;i<N;i++){
          A[i] = sc.nextInt();
          gA[i] = 1/A[i];
          num = num + gA[i];
       	}
      
      	num = 1/num;
		System.out.println(num);
	}
} import java.util.*; util.* java public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int N = sc.nextInt();
      
	    double[] A = new double[N];
	    double[] gA = new double[N];
	    double num = 0;
    
  	    for(int i = 0 ;i<N;i++){
          A[i] = sc.nextInt();
          gA[i] = 1/A[i];
          num = num + gA[i];
       	}
      
      	num = 1/num;
		System.out.println(num);
	}
} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int N = sc.nextInt();
      
	    double[] A = new double[N];
	    double[] gA = new double[N];
	    double num = 0;
    
  	    for(int i = 0 ;i<N;i++){
          A[i] = sc.nextInt();
          gA[i] = 1/A[i];
          num = num + gA[i];
       	}
      
      	num = 1/num;
		System.out.println(num);
	} main {
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int N = sc.nextInt();
      
	    double[] A = new double[N];
	    double[] gA = new double[N];
	    double num = 0;
    
  	    for(int i = 0 ;i<N;i++){
          A[i] = sc.nextInt();
          gA[i] = 1/A[i];
          num = num + gA[i];
       	}
      
      	num = 1/num;
		System.out.println(num);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc double[] A = new double[N]; A new double[N] N double[] gA = new double[N]; gA new double[N] N double num = 0; num 0 for(int i = 0 ;i<N;i++){
          A[i] = sc.nextInt();
          gA[i] = 1/A[i];
          num = num + gA[i];
       	} int i = 0 ; int i = 0 ; i 0 i<N i N i++ i++ i {
          A[i] = sc.nextInt();
          gA[i] = 1/A[i];
          num = num + gA[i];
       	} {
          A[i] = sc.nextInt();
          gA[i] = 1/A[i];
          num = num + gA[i];
       	} A[i] = sc.nextInt() A[i] A i sc.nextInt() sc.nextInt sc gA[i] = 1/A[i] gA[i] gA i 1/A[i] 1 A[i] A i num = num + gA[i] num num + gA[i] num gA[i] gA i num = 1/num num 1/num 1 num System.out.println(num) System.out.println System.out System System.out num String[] args args public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int N = sc.nextInt();
      
	    double[] A = new double[N];
	    double[] gA = new double[N];
	    double num = 0;
    
  	    for(int i = 0 ;i<N;i++){
          A[i] = sc.nextInt();
          gA[i] = 1/A[i];
          num = num + gA[i];
       	}
      
      	num = 1/num;
		System.out.println(num);
	}
} public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int N = sc.nextInt();
      
	    double[] A = new double[N];
	    double[] gA = new double[N];
	    double num = 0;
    
  	    for(int i = 0 ;i<N;i++){
          A[i] = sc.nextInt();
          gA[i] = 1/A[i];
          num = num + gA[i];
       	}
      
      	num = 1/num;
		System.out.println(num);
	}
} Main
import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int N = sc.nextInt();
      
	    double[] A = new double[N];
	    double[] gA = new double[N];
	    double num = 0;
    
  	    for(int i = 0 ;i<N;i++){
          A[i] = sc.nextInt();
          gA[i] = 1/A[i];
          num = num + gA[i];
       	}
      
      	num = 1/num;
		System.out.println(num);
	}
}