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, m = map(int, raw_input().split())
s = raw_input()
t = raw_input()
k = 0
for i in range(n):
	k += s[i] != t[i]
if m * 2 < k:
	print '-1'
	exit(0)
t1 = min(m, k)
p = max(0, m - t1)
t2 = t1 = k - t1
ss = ''
for i in range(n):
	if s[i] == t[i]:
		if p:
			for c in 'ab':
				if c != s[i]:
					ss += c
					break
			p -= 1
		else:
			ss += s[i]
	else:
		if t1:
			ss += s[i]
			t1 -= 1
		elif t2:
			ss += t[i]
			t2 -= 1
		else:
			for c in 'abc':
				if c != s[i] and c != t[i]:
					ss += c
					break
print ss