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Lucky Conversion

题面翻译

定义幸运数字为只含有“幸运字符”（4和7）的正整数。如4，7，744是，而5,17,467不是。

现有两个等长的字符串a，b，只含“幸运字符”，对a进行下面两个操作中的任意一种：

1）将任意一个a中的字符从4变7，或者从7变4；

2）将a中任意两个字符位置互换。

求能使a变成b的最小操作次数。

题目描述

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.

输入格式

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 $10^{5}$ .

输出格式

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

样例 #1

样例输入 #1

47
74

样例输出 #1

1

样例 #2

样例输入 #2

774
744

样例输出 #2

1

样例 #3

样例输入 #3

777
444

样例输出 #3

3

提示

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.