import java.util.Scanner;
public class Main {

	public static void main(String[] args) {
		Scanner scanner=new Scanner(System.in);
		int N=scanner.nextInt();
		int[]arr=new int[N];
		for(int index=0;index<N;index++) {
			arr[index]=scanner.nextInt();
		}
		float sum=0;
		for(int value:arr) {
			sum+=1.0/value;
		}
		System.out.println(1.0*Math.round(100000/sum)/100000);
	}
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz