CF145A.Lucky Conversion

Petya loves lucky numbers very much. Everybody knows that lucky numbers are positive integers whose decimal record contains only the lucky digits 4 and 7. For example, numbers 47, 744, 4 are lucky and 5, 17, 467 are not.

Petya has two strings a and b of the same length n. The strings consist only of lucky digits. Petya can perform operations of two types:

• replace any one digit from string a by its opposite (i.e., replace 4 by 7 and 7 by 4);
• swap any pair of digits in string a.

Petya is interested in the minimum number of operations that are needed to make string a equal to string b. Help him with the task.

佩蒂亚非常喜爱幸运数字。众所周知，幸运数字是指十进制表示中仅包含幸运数字 4 和 7 的正整数。例如，47、744、4 是幸运数字，而 5、17、467 则不是。

佩蒂亚有两个长度均为 $ n $ 的字符串 $ a $ 和 $ b $，且这两个字符串仅由幸运数字组成。佩蒂亚可以执行以下两种操作：

• 将字符串 $ a $ 中的任意一位数字替换为其相反数字（即把 4 替换为 7，或把 7 替换为 4）；
• 交换字符串 $ a $ 中任意两个数字的位置。

佩蒂亚希望知道：使字符串 $ a $ 变为字符串 $ b $ 所需的最少操作次数是多少？请帮助他解决这个问题。

The first and the second line contains strings a and b, correspondingly. Strings a and b have equal lengths and contain only lucky digits. The strings are not empty, their length does not exceed 105.

第一行和第二行分别包含字符串 $a$ 和 $b$。字符串 $a$ 和 $b$ 长度相等，且仅由幸运数字组成。字符串非空，其长度不超过 $10^5$。

Print on the single line the single number — the minimum number of operations needed to convert string a into string b.

在单行上输出一个整数——将字符串 $a$ 转换为字符串 $b$ 所需的最少操作次数。

In the first sample it is enough simply to swap the first and the second digit.

In the second sample we should replace the second digit with its opposite.

In the third number we should replace all three digits with their opposites.

在第一个样例中，只需交换第一位和第二位数字即可。

在第二个样例中，应将第二位数字替换为其相反数。

在第三个数字中，应将全部三位数字均替换为其相反数。