Courses

courses, nl, command = {}, '\n-- ', input()
while True:
    if command == "end":
        break
    course, student = command.split(" : ")
    if course not in courses:
        courses[course] = []
    courses[course].append(student)
    command = input()
[print(f"{k}: {len(v)}\n-- {nl.join(sorted(v))}")
 for k, v in {k: v for k, v in sorted(courses.items(), key=lambda x: -len(x[1]))}.items()]