import java.util.Scanner;
public class Main {
	public static void main(String... args){
      Scanner sc = new Scanner(System.in);
      int N = sc.nextInt();
      double sum = 0.0;
      for(int i = 0; i < N; i++) {
        int input = sc.nextInt();
        sum += (double) (1 / (double)input);
      }
      sum = 1 / sum;
      System.out.println(sum);
      
    }
}
/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