07. Courses

collected_data = {}
input_data = input()

while not input_data == "end":
    course, student = input_data.split(" : ")
    if course not in collected_data:
        collected_data[course] = []
    collected_data[course].append(student)
    input_data = input()

collected_data = sorted(collected_data.items(), key=lambda kvp: -len(kvp[1]))

for courses, students in collected_data:
    print(f"{courses}: {len(students)}")
    students = sorted(students)
    for current_student in students:
        print(f"-- {current_student}")