import java.util.Scanner;

class Main {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int n = s.nextInt();
		int a[] = new int[n];
		
		double ans = 0;
		for(int i=0;i<n;i++) {
			a[i] = s.nextInt();
			ans+=(1.0/a[i]);
		}
		
		System.out.format("%.10f",1.0/ans);
		
		
		
		
	}
} import java.util.Scanner; Scanner java.util class Main {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int n = s.nextInt();
		int a[] = new int[n];
		
		double ans = 0;
		for(int i=0;i<n;i++) {
			a[i] = s.nextInt();
			ans+=(1.0/a[i]);
		}
		
		System.out.format("%.10f",1.0/ans);
		
		
		
		
	}
} Main public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int n = s.nextInt();
		int a[] = new int[n];
		
		double ans = 0;
		for(int i=0;i<n;i++) {
			a[i] = s.nextInt();
			ans+=(1.0/a[i]);
		}
		
		System.out.format("%.10f",1.0/ans);
		
		
		
		
	} main {
		Scanner s = new Scanner(System.in);
		int n = s.nextInt();
		int a[] = new int[n];
		
		double ans = 0;
		for(int i=0;i<n;i++) {
			a[i] = s.nextInt();
			ans+=(1.0/a[i]);
		}
		
		System.out.format("%.10f",1.0/ans);
		
		
		
		
	} Scanner s = new Scanner(System.in); s new Scanner(System.in) int n = s.nextInt(); n s.nextInt() s.nextInt s int a[] = new int[n]; a new int[n] n double ans = 0; ans 0 for(int i=0;i<n;i++) {
			a[i] = s.nextInt();
			ans+=(1.0/a[i]);
		} int i=0; int i=0; i 0 i<n i n i++ i++ i {
			a[i] = s.nextInt();
			ans+=(1.0/a[i]);
		} {
			a[i] = s.nextInt();
			ans+=(1.0/a[i]);
		} a[i] = s.nextInt() a[i] a i s.nextInt() s.nextInt s ans+=(1.0/a[i]) ans (1.0/a[i]) 1.0 a[i] a i System.out.format("%.10f",1.0/ans) System.out.format System.out System System.out "%.10f" 1.0/ans 1.0 ans String[] args args
import java.util.Scanner;

class Main {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int n = s.nextInt();
		int a[] = new int[n];
		
		double ans = 0;
		for(int i=0;i<n;i++) {
			a[i] = s.nextInt();
			ans+=(1.0/a[i]);
		}
		
		System.out.format("%.10f",1.0/ans);
		
		
		
		
	}
}
