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:
# -*- coding: utf-8 -*-
import sys,math,heapq,itertools as it,fractions,re,bisect,collections as coll

alpha = "abcdefghijklmnopqrstuvwxyz"
def another(a1, a2):
    for a in alpha:
        if a != a1 and a != a2:
            return a

n, t = map(int, raw_input().split())
s1 = raw_input()
s2 = raw_input()

ans = [another(a1, a2) for a1, a2 in zip(s1, s2)]
t1, t2 = n, n
for i in xrange(n):
    if t1 == t2 == t: break
    if s1[i] == s2[i]:
        ans[i] = s1[i]
        t1 -= 1
        t2 -= 1

i = 0
while i < n:
    if t1 == t: break
    if ans[i] != s1[i]:
        ans[i] = s1[i]
        t1 -= 1
    i += 1

while i < n:
    if t2 == t: break
    if ans[i] != s2[i]:
        ans[i] = s2[i]
        t2 -= 1
    i += 1

if t1 == t2 == t:
    print "".join(ans)
else:
    print -1