def span(string, char):
    i = 0
    while i < len(string) and string[i] == char:
        i += 1
    return string[0:i], string[i:]


def count_occurrences(string):
    if string:
        print(len(span(string, string[0])[0]), string[0], sep='', end='')
        count_occurrences(span(string, string[0])[1])
    else:
        print()

count_occurrences('aaaaabbbbccccccaaaaaaa')