A9990. Marina and Vasya

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 $t$ ( $1<=n<=10^{5}$ , $0<=t<=n$ ).

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.