I hesitated to buy it because there were few reviews, but I was so happy that the concepts I had built up by searching the internet for a month came out in the first 10 minutes. If I had listened to it earlier, I could have saved a lot of time ㅠㅠㅠ I regret it. As one of the reviewers said, you need to listen to it when you have some understanding of the concepts to be able to hear it more clearly. You can hear the key points explained step by step.