human: 生成一个正确的Python程序，将给定单词中的所有字母向前移动两个字符。 输入单词：apple
 bot: def shift_letters(word):
    shifted_word = ''
    for letter in word:
        shifted_letter = chr(ord(letter) - 2)
        shifted_word += shifted_letter
    return shifted_word

word = input('请输入一个单词：')
shifted_word = shift_letters(word)
print(shifted_word)