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 PYTHON3):

Solution:
def other(c1, c2):
	if c1 != 'z' and c2 != 'z':
		return('z')
	elif c1 != 'y' and c2 != 'y':
		return('y')
	else:
		return('x')

n, t = map(int, input().split())

s1 = input()
s2 = input()

f = sum((1 if c1 != c2 else 0) for (c1, c2) in zip(s1, s2))
if f > 2 * t:
	print(-1)
elif f >= t:
	result = []
	diff = 0
	for c1, c2 in zip(s1, s2):
		if c1 == c2:
			result.append(c1)
		else:
			if diff < f - t:
				result.append(c2)
			elif diff < t:
				result.append(other(c1, c2))
			else:
				result.append(c1)
			diff += 1
	print(''.join(result))
else:
	result = []
	usedeq = 0
	for c1, c2 in zip(s1, s2):
		if c1 == c2:
			if usedeq < (t - f):
				result.append(other(c1, c1))
				usedeq += 1
			else:
				result.append(c1)
		else:
			result.append(other(c1, c2))
	print(''.join(result))