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:
def main():
    import sys
    
    n, t, s1, s2 = sys.stdin.read().split()
    n, t = int(n), int(t)
    
    result = [-1] * n
    rest = n - t
    for i in range(n):
        if rest == 0: break
        if s1[i] == s2[i]:
            result[i] = s1[i]
            rest -= 1
    k = rest
    for i in range(n):
        if k == 0: break
        if result[i] == -1:
            result[i] = s1[i]
            k -= 1
    k = rest
    for i in range(n):
        if k == 0: break
        if result[i] == -1:
            result[i] = s2[i]
            k -= 1    
    if k > 0:
        print(-1)
        return
    for i in range(n):
        if result[i] == -1:
            for j in range(ord('a'), ord('a') + 4):
                if chr(j) != s1[i] and chr(j) != s2[i]:
                    result[i] = chr(j)
                    break
    
    sys.stdout.write(''.join(result))
    
main()