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:
n, t = map(int, input().split())
a, b = input(), input()

ans = []
s = d = 0

for i, j in zip(a, b):
	if i == j:
		s += 1
	else:
		d += 1

if t < (d + 1) // 2:
	print(-1)
	exit()

x = min(n - t, s)
y = z = max(n - t - s, 0)

def f(a, b):
	for i in range(97, 123):
		if chr(i) not in [a, b]:
			return chr(i)


for i in range(n):
	if a[i] == b[i]:
		if x:
			ans.append(a[i])
			x -= 1
		else:
			ans.append(f(a[i], b[i]))
	else:
		if y:
			ans.append(a[i])
			y -= 1
		elif z:
			ans.append(b[i])
			z -= 1
		else:
			ans.append(f(a[i], b[i]))

print(''.join(ans))