import java.util.*;
import java.lang.*;
class Main{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
      	double num = 0.0;
      	int child = 1;
     	int mom = 1;
     	for(int i = 0; i < a; i++){
          num += 1.0/sc.nextInt();
        }
        System.out.println(1.0/num);
    }
}
/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