def character_frequency(words):
    words = words.upper().split(' ')
    counts = [0] * 26
    for word in words:
        for char in word:
            counts[ord(char) - ord('A')] += 1

    letters = [chr(x+ord('A')) for x in range(26)]
    return sorted(zip(counts, letters))[-1: -11: -1]