import java.util.Scanner;
public class Main {

	public static void main(String[] args) {
		Scanner st=new Scanner(System.in);
		int n=st.nextInt();
		int[] a=new int[n];
		for(int i=0;i<n;i++) {
			a[i]=st.nextInt();
		}
		double kotae=0;
		for(int i=0;i<n;i++) {
			kotae+=1.0/(double)a[i];
		}
		double kotae1=0.0;
		kotae1=1.0/kotae;
		System.out.printf("%f",kotae1);

	}

} import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {
		Scanner st=new Scanner(System.in);
		int n=st.nextInt();
		int[] a=new int[n];
		for(int i=0;i<n;i++) {
			a[i]=st.nextInt();
		}
		double kotae=0;
		for(int i=0;i<n;i++) {
			kotae+=1.0/(double)a[i];
		}
		double kotae1=0.0;
		kotae1=1.0/kotae;
		System.out.printf("%f",kotae1);

	}

} Main public static void main(String[] args) {
		Scanner st=new Scanner(System.in);
		int n=st.nextInt();
		int[] a=new int[n];
		for(int i=0;i<n;i++) {
			a[i]=st.nextInt();
		}
		double kotae=0;
		for(int i=0;i<n;i++) {
			kotae+=1.0/(double)a[i];
		}
		double kotae1=0.0;
		kotae1=1.0/kotae;
		System.out.printf("%f",kotae1);

	} main {
		Scanner st=new Scanner(System.in);
		int n=st.nextInt();
		int[] a=new int[n];
		for(int i=0;i<n;i++) {
			a[i]=st.nextInt();
		}
		double kotae=0;
		for(int i=0;i<n;i++) {
			kotae+=1.0/(double)a[i];
		}
		double kotae1=0.0;
		kotae1=1.0/kotae;
		System.out.printf("%f",kotae1);

	} Scanner st=new Scanner(System.in); st new Scanner(System.in) int n=st.nextInt(); n st.nextInt() st.nextInt st int[] a=new int[n]; a new int[n] n for(int i=0;i<n;i++) {
			a[i]=st.nextInt();
		} int i=0; int i=0; i 0 i<n i n i++ i++ i {
			a[i]=st.nextInt();
		} {
			a[i]=st.nextInt();
		} a[i]=st.nextInt() a[i] a i st.nextInt() st.nextInt st double kotae=0; kotae 0 for(int i=0;i<n;i++) {
			kotae+=1.0/(double)a[i];
		} int i=0; int i=0; i 0 i<n i n i++ i++ i {
			kotae+=1.0/(double)a[i];
		} {
			kotae+=1.0/(double)a[i];
		} kotae+=1.0/(double)a[i] kotae 1.0/(double)a[i] 1.0 (double)a[i] a i double kotae1=0.0; kotae1 0.0 kotae1=1.0/kotae kotae1 1.0/kotae 1.0 kotae System.out.printf("%f",kotae1) System.out.printf System.out System System.out "%f" kotae1 String[] args args public class Main {

	public static void main(String[] args) {
		Scanner st=new Scanner(System.in);
		int n=st.nextInt();
		int[] a=new int[n];
		for(int i=0;i<n;i++) {
			a[i]=st.nextInt();
		}
		double kotae=0;
		for(int i=0;i<n;i++) {
			kotae+=1.0/(double)a[i];
		}
		double kotae1=0.0;
		kotae1=1.0/kotae;
		System.out.printf("%f",kotae1);

	}

} public class Main {

	public static void main(String[] args) {
		Scanner st=new Scanner(System.in);
		int n=st.nextInt();
		int[] a=new int[n];
		for(int i=0;i<n;i++) {
			a[i]=st.nextInt();
		}
		double kotae=0;
		for(int i=0;i<n;i++) {
			kotae+=1.0/(double)a[i];
		}
		double kotae1=0.0;
		kotae1=1.0/kotae;
		System.out.printf("%f",kotae1);

	}

} Main
import java.util.Scanner;
public class Main {

	public static void main(String[] args) {
		Scanner st=new Scanner(System.in);
		int n=st.nextInt();
		int[] a=new int[n];
		for(int i=0;i<n;i++) {
			a[i]=st.nextInt();
		}
		double kotae=0;
		for(int i=0;i<n;i++) {
			kotae+=1.0/(double)a[i];
		}
		double kotae1=0.0;
		kotae1=1.0/kotae;
		System.out.printf("%f",kotae1);

	}

}
