C. Swap Letters
time limit per test
2 seconds
memory limit per test
256 megabytes
input
standard input
output
standard output

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

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.

Examples
Input
4
abab
aabb

Output
2
3 3
3 2

Input
1
a
b

Output
-1

Input
8
babbaabb
abababaa

Output
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.