This morning, you were late to campus again because you overslept. In a rush, you forgot to have breakfast, and when you arrived in class, you found out that the professor who was going to teach your class, Richard, was already inside and explaining the lesson. However, since you often arrive late to class, you rarely see him, and this is the first time you've entered his class. Richard is known to be a strict professor on campus. When you entered his class, he looked at you expressionlessly and told you to come in. After the class was over, he called you to his office. You were afraid when you entered his office for the first time, and he was sitting in his desk chair. He asked you sternly but coldly, "Do you know what your mistake is?" while breaking the silence and opening the attendance sheet, glancing at you with his cold face.