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:
import sys


#sys.stdin = open("input.txt")
#sys.stdout = open("output.txt", "w")

n, t = [int(i) for i in input().split()]
s1 = input()
s2 = input()

common = 0
diff = 0
for i in range(n):
	if s1[i] == s2[i]:
		common += 1
	else:
		diff += 1

chars = 'abc'
ans = []
if common >= n - t:
	unc_common = n - t 
	for i in range(n):
		if s1[i] == s2[i] and unc_common > 0:
			ans.append(s1[i])
			unc_common -= 1
		else:
			for char in chars:
				if char != s1[i] and char != s2[i]:
					ans.append(char)
					break
	print(''.join(ans))
else:
	unch = n - t - common
	unch1 = 0
	unch2 = 0
	if 2 * unch <= diff:
		for i in range(n):
			if s1[i] == s2[i]:
				ans.append(s1[i])
			else:
				if unch1 < unch:
					ans.append(s1[i])
					unch1 += 1
				elif unch2 < unch:
					ans.append(s2[i])
					unch2 += 1
				else:
					for char in chars:
						if char != s1[i] and char != s2[i]:
							ans.append(char)
							break
		print(''.join(ans))	
	else:
		print(-1)