Swap Letters
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.
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.
Input
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".
Output
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.
Samples
4
abab
aabb
2
3 3
3 2
1
a
b
-1
8
babbaabb
abababaa
3
2 6
1 3
7 8
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.