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:
#!/bin/sh
# -*- coding: utf-8 -*-
''''which python2 >/dev/null && exec python2 "$0" "$@" # '''
''''which python  >/dev/null && exec python  "$0" "$@" # '''

import string
import sys

n, t = map(int, raw_input().split())
t = n-t
s1 = raw_input()
s2 = raw_input()
s = sum(map(lambda x: 1 if x[0]==x[1] else 0, zip(s1, s2)))

if t <= s:
    for a, b in zip(s1, s2):
        if t and a==b:
            sys.stdout.write(a)
            t -= 1
        else:
            if a != 'a' and b != 'a':
                sys.stdout.write('a')
            elif a != 'b'and b != 'b':
                sys.stdout.write('b')
            else:
                sys.stdout.write('c')
    print
elif (n-s)>=(t-s)*2:
    cnt1 = 0
    cnt2 = 0
    for a, b in zip(s1, s2):
        if a == b:
            sys.stdout.write(a)
        else:
            if cnt1 < t-s:
                sys.stdout.write(a)
                cnt1 += 1
            elif cnt2 < t-s:
                sys.stdout.write(b)
                cnt2 += 1
            else:
                if a != 'a' and b != 'a':
                    sys.stdout.write('a')
                elif a != 'b'and b != 'b':
                    sys.stdout.write('b')
                else:
                    sys.stdout.write('c')
    print
else:
    print -1