import java.util.Scanner;
public class Main {
    public static void main(String[] args){
        Scanner scan = new Scanner(System.in);
        final int inputNum = scan.nextInt();
        int[] values = new int[inputNum];
        float math = (float)0.0;
        for(int i = 0; i < inputNum; i++){
            values[i] = scan.nextInt();
        }
        for(int i = 0; i < inputNum; i++){
            math += (float)1 / values[i];
        }
        System.out.println((float)1 / math);
        
        scan.close();
    }
  
}
/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