B'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 <= n <= 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. 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. '...