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:
#!/usr/bin/env python
# -*- coding: utf-8 -*-

n,t = map(int,input().split())
s1 = list(input())
s2 = list(input())
lowercase = set([chr(i) for i in range(ord('a'), ord('z')+1)]) 

ans = [False for i in range(n)]
count = 0

for i in range(n):
    if s1[i] != s2[i]:
        count += 1
    else:
        ans[i] = s1[i]

if count > t and count < (count-t) * 2:
    print(-1)
else:
    for i in range(n):
        if count > t and not ans[i]:
            ans[i] = s1[i]
            for j in range(i,n):
                if not ans[j]:
                    ans[j] = s2[j]
                    break
            count -= 1
        elif count < t and ans[i] is not False:
            ans[i] = (lowercase - set(s1[i]) - set(s2[i])).pop()
            count += 1
        elif not ans[i]:
            ans[i] = (lowercase - set(s1[i]) - set(s2[i])).pop()
    print(''.join(ans))