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:
#  =========     /\       /|    |====/|
#      |        /  \       |    |   / |
#      |       /____\      |    |  /  |
#      |      /      \     |    | /   |
#  ========= /        \  =====  |/====|  
#  code

def main():

    def getD(c1, c2):
        for i in [chr(i) for i in range(ord('a'),ord('z') + 1)]:
            if i != c1 and i != c2:
                return i

    n, t = map(int, input().split())
    s1 = input()
    s2 = input()
    s3 = ['-'] * n

    same = 0

    for i, v in enumerate(s1):
        if v == s2[i]:
            same += 1
    
    if n - t <= same:
        cnt = n - t
        for i, v in enumerate(s1):
            if cnt <= 0:
                break
            if v == s2[i]:
                s3[i] = v
                cnt -= 1
        
        for i, v in enumerate(s3):
            if v == '-':
                s3[i] = getD(s1[i], s2[i])
        
        print(''.join(s3))
    else:
        cnt = n - t
        for i, v in enumerate(s1):
            if v == s2[i]:
                s3[i] = v
                cnt -= 1
        
        cnt1 = cnt
        for i, v in enumerate(s3):
            if cnt1 <= 0:
                break
            if v == '-':
                s3[i] = s1[i]
                cnt1 -= 1
        
        if cnt1 > 0:
            print(-1)
            return

        cnt2 = cnt
        for i, v in enumerate(s3):
            if cnt2 <= 0:
                break
            if v == '-':
                s3[i] = s2[i]
                cnt2 -= 1
        
        if cnt2:
            print(-1)
            return
        
        for i, v in enumerate(s3):
            if v == '-':
                s3[i] = getD(s1[i], s2[i])

        print(''.join(s3))
    return

if __name__ == "__main__":
    main()