import java.util.*;

public class Main
{
	public static void main(String [] Args)
	{
		Scanner in = new Scanner(System.in);
		int n=in.nextInt();
		double ans=0.0;
		for(int i=0;i<n;i++)
		{
			double q=in.nextDouble();
			ans=ans+1.0/q;
		}
		ans=1.0/ans;
		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