import java.util.*;

public class Main {
	static int N=26;
	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		N = sc.nextInt();
		int data[] =new int[N];
		double t=0;
	
		for(int i=0;i<data.length;i++) {
			data[i]=sc.nextInt();
			
			t += (double)1.0/data[i];
		}
		double ans =( double)1.0/t;
	System.out.println(ans);

		

	}

}
/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