以下為此題暴力解法

a, b = map(int, input().split())
student_ids = []
student_ids_arrive = []

for i in range(a):
    student_id = input()
    student_ids.append(student_id)

for i in range(b):
    student_id_arrive = input()
    student_ids_arrive.append(student_id_arrive)
print(a-b)