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:
#!/usr/bin/env python
from string import ascii_lowercase

def other(c1, c2):
    if 'a' not in (c1, c2):
        return 'a'
    if 'b' not in (c1, c2):
        return 'b'
    if 'c' not in (c1, c2):
        return 'c'

[n,t] = map(int, raw_input().split(' '))

s1 = raw_input()
s2 = raw_input()

equal = n-t

s3 = ""

c = 0
for a,b in zip(s1,s2):
    if a == b:
        c +=1
#    s3='a' if a!='a' else 'b'


if c >= equal:
    e = 0
    i = 0
    for a,b in zip(s1,s2):
        if a == b and e<equal:
            e+=1
            s3 += a
        else:
            s3 += other(a,b)
    print s3
else:
    change = equal-c
    direction = True
    i = 0
    for a,b in zip(s1,s2):
        if a == b:
            s3 += a
        elif change > 0:
            if direction:
                s3 += a
            else:
                s3 += b
                change-=1
            direction = not direction
        else:
            s3 += other(a,b)
    if change > 0:
        print -1
    else:
        print s3