import java.util.Scanner;
public class Main {
	
		static void solve() {
			Scanner sc = new Scanner(System.in);
			int n = sc.nextInt();
			double res = 0.0;
			for(int i = 0;i<n;i++) {
				res+=1.0/sc.nextDouble();
			}
			System.out.print(1.0/res);
			sc.close();
		}
	
	public static void main(String[] args){
		solve();
	}
} import java.util.Scanner; Scanner java.util public class Main {
	
		static void solve() {
			Scanner sc = new Scanner(System.in);
			int n = sc.nextInt();
			double res = 0.0;
			for(int i = 0;i<n;i++) {
				res+=1.0/sc.nextDouble();
			}
			System.out.print(1.0/res);
			sc.close();
		}
	
	public static void main(String[] args){
		solve();
	}
} Main static void solve() {
			Scanner sc = new Scanner(System.in);
			int n = sc.nextInt();
			double res = 0.0;
			for(int i = 0;i<n;i++) {
				res+=1.0/sc.nextDouble();
			}
			System.out.print(1.0/res);
			sc.close();
		} solve {
			Scanner sc = new Scanner(System.in);
			int n = sc.nextInt();
			double res = 0.0;
			for(int i = 0;i<n;i++) {
				res+=1.0/sc.nextDouble();
			}
			System.out.print(1.0/res);
			sc.close();
		} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc double res = 0.0; res 0.0 for(int i = 0;i<n;i++) {
				res+=1.0/sc.nextDouble();
			} int i = 0; int i = 0; i 0 i<n i n i++ i++ i {
				res+=1.0/sc.nextDouble();
			} {
				res+=1.0/sc.nextDouble();
			} res+=1.0/sc.nextDouble() res 1.0/sc.nextDouble() 1.0 sc.nextDouble() sc.nextDouble sc System.out.print(1.0/res) System.out.print System.out System System.out 1.0/res 1.0 res sc.close() sc.close sc public static void main(String[] args){
		solve();
	} main {
		solve();
	} solve() solve String[] args args public class Main {
	
		static void solve() {
			Scanner sc = new Scanner(System.in);
			int n = sc.nextInt();
			double res = 0.0;
			for(int i = 0;i<n;i++) {
				res+=1.0/sc.nextDouble();
			}
			System.out.print(1.0/res);
			sc.close();
		}
	
	public static void main(String[] args){
		solve();
	}
} public class Main {
	
		static void solve() {
			Scanner sc = new Scanner(System.in);
			int n = sc.nextInt();
			double res = 0.0;
			for(int i = 0;i<n;i++) {
				res+=1.0/sc.nextDouble();
			}
			System.out.print(1.0/res);
			sc.close();
		}
	
	public static void main(String[] args){
		solve();
	}
} Main
import java.util.Scanner;
public class Main {
	
		static void solve() {
			Scanner sc = new Scanner(System.in);
			int n = sc.nextInt();
			double res = 0.0;
			for(int i = 0;i<n;i++) {
				res+=1.0/sc.nextDouble();
			}
			System.out.print(1.0/res);
			sc.close();
		}
	
	public static void main(String[] args){
		solve();
	}
}
