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:
'''input
4 2
acbb
babc
'''
from sys import stdin


def get_different(a, b):
	for i in range(97, 97 + 26):
		if chr(i) not in [a, b]:
			return chr(i)


# main starts
n, t = list(map(int,stdin.readline().split()))
first = stdin.readline().strip()
second = stdin.readline().strip()

diff = 0
same = 0
for i in range(n):
	if first[i] == second[i]:
		same += 1
	else:
		diff += 1

if diff % 2 == 0:
	mn = diff//2
else:
	mn = diff//2 + 1

if t < mn:
	print(-1)
	exit()

ans = [0] * n
# processing different
i = 0
flag = True
while diff > t and i < n:
	if first[i] != second[i]:
		if flag:
			ans[i] = first[i]
			flag = False
		else:
			ans[i] = second[i]
			flag = True
			diff -= 2
			t -= 1
	i += 1
# print(ans)

i = 0
while t > 0 and diff > 0 and i < n:
	if first[i] != second[i] and ans[i] == 0:
		ans[i] = get_different(first[i], second[i])
		diff -= 1
		t -= 1
	i += 1

i = 0
while t > 0 and i < n:
	if first[i] == second[i] and ans[i] == 0:
		ans[i] = get_different(first[i], first[i])
		same -= 1
		t -= 1
	i += 1

for i in range(n):
	if ans[i] == 0:
		ans[i] = first[i]

print(''.join(ans))