|Codeforces Round #324 (Div. 2)|
Marina loves strings of the same length and Vasya loves when there is a third string, different from them in exactly t characters. Help Vasya find at least one such string.
More formally, you are given two strings s 1, s 2 of length n and number t. Let's denote as f(a, b) the number of characters in which strings a and b are different. Then your task will be to find any string s 3 of length n, such that f(s 1, s 3) = f(s 2, s 3) = t. If there is no such string, print - 1.
The first line contains two integers n and t (1 ≤ n ≤ 105, 0 ≤ t ≤ n).
The second line contains string s 1 of length n, consisting of lowercase English letters.
The third line contain string s 2 of length n, consisting of lowercase English letters.
Print a string of length n, differing from string s 1 and from s 2 in exactly t characters. Your string should consist only from lowercase English letters. If such string doesn't exist, print -1.