A8457 | 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.

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 $10^{5}$ .

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