import java.util.*;
public class Main{
    public static void main(String[] args){
        Scanner sc=new Scanner(System.in);
        int N=sc.nextInt();
        int A[];
        A=new int[N];
        for(int i=0;i<N;i++) A[i]=sc.nextInt();
        double ans=0;
        for(int i=0;i<N;i++) ans+=(double)1/A[i];
        ans=1/ans;
        System.out.printf("%.10f\n",ans);
    }
} 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[];
        A=new int[N];
        for(int i=0;i<N;i++) A[i]=sc.nextInt();
        double ans=0;
        for(int i=0;i<N;i++) ans+=(double)1/A[i];
        ans=1/ans;
        System.out.printf("%.10f\n",ans);
    }
} Main public static void main(String[] args){
        Scanner sc=new Scanner(System.in);
        int N=sc.nextInt();
        int A[];
        A=new int[N];
        for(int i=0;i<N;i++) A[i]=sc.nextInt();
        double ans=0;
        for(int i=0;i<N;i++) ans+=(double)1/A[i];
        ans=1/ans;
        System.out.printf("%.10f\n",ans);
    } main {
        Scanner sc=new Scanner(System.in);
        int N=sc.nextInt();
        int A[];
        A=new int[N];
        for(int i=0;i<N;i++) A[i]=sc.nextInt();
        double ans=0;
        for(int i=0;i<N;i++) ans+=(double)1/A[i];
        ans=1/ans;
        System.out.printf("%.10f\n",ans);
    } Scanner sc=new Scanner(System.in); sc new Scanner(System.in) int N=sc.nextInt(); N sc.nextInt() sc.nextInt sc int A[]; A 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 double ans=0; ans 0 for(int i=0;i<N;i++) ans+=(double)1/A[i]; int i=0; int i=0; i 0 i<N i N i++ i++ i ans+=(double)1/A[i]; ans+=(double)1/A[i] ans (double)1/A[i] (double)1 A[i] A i ans=1/ans ans 1/ans 1 ans System.out.printf("%.10f\n",ans) System.out.printf System.out System System.out "%.10f\n" ans String[] args args public class Main{
    public static void main(String[] args){
        Scanner sc=new Scanner(System.in);
        int N=sc.nextInt();
        int A[];
        A=new int[N];
        for(int i=0;i<N;i++) A[i]=sc.nextInt();
        double ans=0;
        for(int i=0;i<N;i++) ans+=(double)1/A[i];
        ans=1/ans;
        System.out.printf("%.10f\n",ans);
    }
} public class Main{
    public static void main(String[] args){
        Scanner sc=new Scanner(System.in);
        int N=sc.nextInt();
        int A[];
        A=new int[N];
        for(int i=0;i<N;i++) A[i]=sc.nextInt();
        double ans=0;
        for(int i=0;i<N;i++) ans+=(double)1/A[i];
        ans=1/ans;
        System.out.printf("%.10f\n",ans);
    }
} 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[];
        A=new int[N];
        for(int i=0;i<N;i++) A[i]=sc.nextInt();
        double ans=0;
        for(int i=0;i<N;i++) ans+=(double)1/A[i];
        ans=1/ans;
        System.out.printf("%.10f\n",ans);
    }
}