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 PYTHON):

Solution:
import string

__author__ = 'trunghieu11'


def getDiff(a, b):
    for c in string.ascii_lowercase:
        if c != a and c != b:
            return c
    return a


def solve(n, k, a, b):
    totalA = k
    totalB = k
    diff = sum(a[i] != b[i] for i in range(n))
    answer = []
    for i in range(n):
        if a[i] != b[i]:
            if totalA == totalB == 0:
                return -1
            if totalA == totalB:
                if totalA + totalB > diff:
                    totalA -= 1
                    totalB -= 1
                    answer.append(getDiff(a[i], b[i]))
                else:
                    totalA -= 1
                    answer.append(b[i])
            else:
                if totalA > totalB:
                    totalA -= 1
                    answer.append(b[i])
                else:
                    totalB -= 1
                    answer.append(a[i])
            diff -= 1
        else:
            answer.append(a[i])
    if totalA + totalB > 0:
        if totalA == totalB:
            for i in range(n):
                if a[i] == b[i] and totalA > 0:
                    answer[i] = getDiff(a[i], b[i])
                    totalA -= 1
        else:
            return -1
    return ''.join(str(x) for x in answer)

if __name__ == '__main__':
    n, k = map(int, raw_input().split(" "))
    a = raw_input()
    b = raw_input()
    print solve(n, k, a, b)