Swap Letters

### 题目描述 ``Monocarp``有两个长度相等的，里面只有字母``a``和``b``的字符串$s,t$。 ``Monocarp``想要使这两个字符串相等。他可以执行以下操作任意次：指定两个下标$pos1,pos2$，交换$s_{pos1}$和$t_{pos2}$。 你需要确定``Monocarp``使这两个串变得相等所需要的最小操作次数，并且输出任意一个这样的步数最少的操作序列。或者说这两个串不可能变得相等。 ### 输入格式 第一行是一个整数$n(1\le n\le 2*10^5)$表示$s,t$的长度。 第二行一个只包含字母``a,b``的长度为$n$的字符串$s$。 第三行一个只包含字母``a,b``的长度为$n$的字符串$t$。 ### 输出格式 如果不可能使这两个串相等，输出``-1``。 否则，第一行输出最小操作步数$k$，然后再输出$k$行，每行两个整数，表示你在这一次进行操作时所指定的$pos1$和$pos2$。 ### 说明/提示 在第一个样例中，你可以交换$s$的第三个字母，在交换$t$的第三个字母。于是$s$变为``abbb``，$t$变为``aaab``。再交换$s$的第三个字母和$t$的第二个字母即可达成目标，使得$s,t$都变成``abab``。 第二个样例中，显然不论怎么交换$s,t$都不可能相等。

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.

The first line contains one integer $ n $ $ (1 \le n \le 2 \cdot 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".

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.

4
abab
aabb

2
3 3
3 2

1
a
b

-1

8
babbaabb
abababaa

3
2 6
1 3
7 8

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.