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:
import string

n, k = map(int, input().split())
s1 = list(input())
s2 = list(input())


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


diffs = 0

for idx in range(n):
    if s1[idx] != s2[idx]:
        diffs += 1

s3 = s1.copy()
if diffs == k:
    for idx in range(n):
        if s1[idx] != s2[idx]:
            s3[idx] = getChar(s1[idx], s2[idx])
    print(''.join(s3))
elif diffs < k:

    for idx in range(n):
        if s1[idx] != s2[idx]:
            s3[idx] = getChar(s1[idx], s2[idx])

    for idx in range(n):
        if s1[idx] == s2[idx]:
            diffs += 1
            s3[idx] = getChar(s1[idx], s2[idx])
            if diffs == k:
                break
    if diffs == k:
        print(''.join(s3))
    else:
        print(-1)
elif diffs > k:
    if 2 * k >= diffs:

        for idx in range(n):
            if s1[idx] != s2[idx]:
                s3[idx] = getChar(s1[idx], s2[idx])

        first = True
        for idx in range(n):
            if s1[idx] != s2[idx]:
                if first:
                    s3[idx] = s1[idx]
                else:
                    s3[idx] = s2[idx]
                    diffs -= 1
                first = not first
                if diffs == k:
                    break
        print(''.join(s3))
    else:
        print(-1)