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:
n,t=map(int,raw_input().split())
t=n-t
t1=t2=t
s1=raw_input().strip()
s2=raw_input().strip()
s3=['.' for i in xrange(n)]
for i in xrange(n):
    if s1[i]==s2[i] and t1:
        t1-=1
        t2-=1
        s3[i]=s1[i]
for i in xrange(n):
    if s3[i]=='.':
        if t1:
            t1-=1
            s3[i]=s1[i]
        elif t2:
            t2-=1
            s3[i]=s2[i]
        else:
            for j in 'abc':
                if j!=s1[i] and j!=s2[i]:s3[i]=j
if t1 or t2:print -1
else:print ''.join(s3)