write a go solution for Description:
Monocarp has got two strings s and t having equal length. Both strings consist of lowercase Latin letters "a" and "b".

Monocarp wants to make these two strings s and t equal to each other. He can do the following operation any number of times: choose an index pos_1 in the string s, choose an index pos_2 in the string t, and swap s_pos_1 with t_pos_2.

You have to determine the minimum number of operations Monocarp has to perform to make s and t equal, and print any optimal sequence of operations — or say that it is impossible to make these strings equal.

Input Format:
The first line contains one integer n (1<=n<=2*10^5) — the length of s and t.

The second line contains one string s consisting of n characters "a" and "b".

The third line contains one string t consisting of n characters "a" and "b".

Output Format:
If it is impossible to make these strings equal, print -1.

Otherwise, in the first line print k — the minimum number of operations required to make the strings equal. In each of the next k lines print two integers — the index in the string s and the index in the string t that should be used in the corresponding swap operation.

Note:
In the first example two operations are enough. For example, you can swap the third letter in s with the third letter in t. Then s= "abbb", t= "aaab". Then swap the third letter in s and the second letter in t. Then both s and t are equal to "abab".

In the second example it's impossible to make two strings equal.. Output only the code with no comments, explanation, or additional text.