Courses

courses = {}

while True:
    course =  input().split(' : ')
    if 'end' in course:
        break
    name = course[0]
    student = course[1]
    if name not in courses:
        courses[name] = [student]
    else:
        courses[name].append(student)

courses_sort = sorted(courses, key=lambda x: len(courses[x]), reverse=True)

for i in courses_sort:
    print(f'{i}: {len(courses[i])}')
    people = sorted(x for x in courses[i])
    [print(f'-- {x}') for x in people]