统计两个长度相等的字符串之间的不同字符个数

def count_different_chars(str1, str2):
    if len(str1) != len(str2):
        return "Error: The lengths of the two strings are not equal."

    different_chars = set()

    for i in range(len(str1)):
        if str1[i] != str2[i]:
            different_chars.add(str1[i])
            different_chars.add(str2[i])

    return len(different_chars)