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