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:
from sys import stdin
def main():
    n, t = map(int, stdin.readline().split())
    s1 = stdin.readline().strip()
    s2 = stdin.readline().strip()
    t = n - t
    ans = list(s1)
    for i in xrange(n):
        if s1[i] == s2[i]:
            if t:
                ans[i] = s1[i]
                t -= 1
            else:
                for k in 'ab':
                    if s1[i] != k:
                        ans[i] = k
                        break
    c1 = c2 = t
    for i in xrange(n):
        if s1[i] != s2[i]:
            if c2 > c1 >= 0:
                ans[i] = s2[i]
                c2 -= 1
            elif c1 == c2 > 0:
                ans[i] = s1[i]
                c1 -= 1
            else:
                for c in 'abc':
                    if s1[i] != c and s2[i] != c:
                        ans[i] = c
                        break
    if c1 + c2 > 0:
        print -1
    else:
        print ''.join(ans)
main()