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