import java.util.Scanner;
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 [] B = new double[N];
		double S =0;
		for(int i=0; i<N; i++) {
			A[i]=sc.nextInt();
		}
		for(int i=0; i<N; i++) {
          	B[i]=1.0/A[i];
			S=S+B[i];
		}
		System.out.println(1.0/S);
	}
}
 import java.util.Scanner; Scanner 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 [] B = new double[N];
		double S =0;
		for(int i=0; i<N; i++) {
			A[i]=sc.nextInt();
		}
		for(int i=0; i<N; i++) {
          	B[i]=1.0/A[i];
			S=S+B[i];
		}
		System.out.println(1.0/S);
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int[] A = new int[N];
		double [] B = new double[N];
		double S =0;
		for(int i=0; i<N; i++) {
			A[i]=sc.nextInt();
		}
		for(int i=0; i<N; i++) {
          	B[i]=1.0/A[i];
			S=S+B[i];
		}
		System.out.println(1.0/S);
	} main {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int[] A = new int[N];
		double [] B = new double[N];
		double S =0;
		for(int i=0; i<N; i++) {
			A[i]=sc.nextInt();
		}
		for(int i=0; i<N; i++) {
          	B[i]=1.0/A[i];
			S=S+B[i];
		}
		System.out.println(1.0/S);
	} 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 [] B = new double[N]; B new double[N] N double S =0; S 0 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 for(int i=0; i<N; i++) {
          	B[i]=1.0/A[i];
			S=S+B[i];
		} int i=0; int i=0; i 0 i<N i N i++ i++ i {
          	B[i]=1.0/A[i];
			S=S+B[i];
		} {
          	B[i]=1.0/A[i];
			S=S+B[i];
		} B[i]=1.0/A[i] B[i] B i 1.0/A[i] 1.0 A[i] A i S=S+B[i] S S+B[i] S B[i] B i System.out.println(1.0/S) System.out.println System.out System System.out 1.0/S 1.0 S 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 [] B = new double[N];
		double S =0;
		for(int i=0; i<N; i++) {
			A[i]=sc.nextInt();
		}
		for(int i=0; i<N; i++) {
          	B[i]=1.0/A[i];
			S=S+B[i];
		}
		System.out.println(1.0/S);
	}
} 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 [] B = new double[N];
		double S =0;
		for(int i=0; i<N; i++) {
			A[i]=sc.nextInt();
		}
		for(int i=0; i<N; i++) {
          	B[i]=1.0/A[i];
			S=S+B[i];
		}
		System.out.println(1.0/S);
	}
} Main
import java.util.Scanner;
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 [] B = new double[N];
		double S =0;
		for(int i=0; i<N; i++) {
			A[i]=sc.nextInt();
		}
		for(int i=0; i<N; i++) {
          	B[i]=1.0/A[i];
			S=S+B[i];
		}
		System.out.println(1.0/S);
	}
}

