import java.util.*;
class Main{
	public static void main(String[] args){
		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();
		}
		double bunsi=1;
		for(int i=0;i<n;i++){
			bunsi*=a[i];
		}
		double bunbo=0;
		for(int i=0;i<n;i++){
			bunbo+=bunsi/a[i];
		}
		System.out.println(bunsi/bunbo);
	}
	
} import java.util.*; util.* java class Main{
	public static void main(String[] args){
		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();
		}
		double bunsi=1;
		for(int i=0;i<n;i++){
			bunsi*=a[i];
		}
		double bunbo=0;
		for(int i=0;i<n;i++){
			bunbo+=bunsi/a[i];
		}
		System.out.println(bunsi/bunbo);
	}
	
} Main public static void main(String[] args){
		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();
		}
		double bunsi=1;
		for(int i=0;i<n;i++){
			bunsi*=a[i];
		}
		double bunbo=0;
		for(int i=0;i<n;i++){
			bunbo+=bunsi/a[i];
		}
		System.out.println(bunsi/bunbo);
	} main {
		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();
		}
		double bunsi=1;
		for(int i=0;i<n;i++){
			bunsi*=a[i];
		}
		double bunbo=0;
		for(int i=0;i<n;i++){
			bunbo+=bunsi/a[i];
		}
		System.out.println(bunsi/bunbo);
	} 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]=sc.nextInt() a[i] a i sc.nextInt() sc.nextInt sc double bunsi=1; bunsi 1 for(int i=0;i<n;i++){
			bunsi*=a[i];
		} int i=0; int i=0; i 0 i<n i n i++ i++ i {
			bunsi*=a[i];
		} {
			bunsi*=a[i];
		} bunsi*=a[i] bunsi a[i] a i double bunbo=0; bunbo 0 for(int i=0;i<n;i++){
			bunbo+=bunsi/a[i];
		} int i=0; int i=0; i 0 i<n i n i++ i++ i {
			bunbo+=bunsi/a[i];
		} {
			bunbo+=bunsi/a[i];
		} bunbo+=bunsi/a[i] bunbo bunsi/a[i] bunsi a[i] a i System.out.println(bunsi/bunbo) System.out.println System.out System System.out bunsi/bunbo bunsi bunbo String[] args args
import java.util.*;
class Main{
	public static void main(String[] args){
		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();
		}
		double bunsi=1;
		for(int i=0;i<n;i++){
			bunsi*=a[i];
		}
		double bunbo=0;
		for(int i=0;i<n;i++){
			bunbo+=bunsi/a[i];
		}
		System.out.println(bunsi/bunbo);
	}
	
}