Problem: 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 s1, s2 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 s3 of length n, such that f(s1, s3) = f(s2, s3) = t. If there is no such string, print  - 1.

Input

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.

Output

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.

Examples

Input

3 2
abc
xyc


Output

ayd

Input

1 0
c
b


Output

-1

Solution (in PYTHON3):

Solution:
__author__ = 'ploskov'


def main():
    n, t = map(int, input().split())
    word1 = input()
    word2 = input()

    different = 0
    result = []
    for i in range(n):
        if word1[i] != word2[i]:
            different += 1
            result.append('_')
        else:
            result.append(word1[i])

    alphabet = [chr(c) for c in range(ord('a'), ord('z') + 1)]
    if t < (different // 2 + different % 2):
        print(-1)
    elif t <= different:
        count1 = different - t
        count2 = different - t
        for index in range(len(result)):
            if result[index] == '_':
                if count1:
                    result[index] = word1[index]
                    count1 -= 1
                    continue
                if count2:
                    result[index] = word2[index]
                    count2 -= 1
                    continue
                for char in alphabet:
                    if char != word1[index] and char != word2[index]:
                        result[index] = char
                        break
        print(''.join(result))
    else:
        t -= different
        for index in range(len(result)):
            if result[index] == '_':
                for char in alphabet:
                    if char != word1[index] and char != word2[index]:
                        result[index] = char
                        break
            elif t:
                for char in alphabet:
                    if char != word1[index] and char != word2[index]:
                        result[index] = char
                        break
                t -= 1
        print(''.join(result))


if __name__ == '__main__':
    main()