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.

输入描述:

The first line contains two integers n and t (1 ≤ n ≤ 105, 0 ≤ t ≤ n).The second line contains string s1 of length n, consisting of lowercase English letters.The third line contain string s2 of length n, consisting of lowercase English letters.

输出描述:

Print a string of length n, differing from string s1 and from s2 in exactly t characters. Your string should consist only from lowercase English letters. If such string doesn't exist, print -1.