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 get_diff(a,b):
	res='a'
	while res==a or res==b:
		res=chr(ord(res)+1)
	return res

n,t=map(int, input().split())
s1=input()
s2=input()
diff=0
res="-1"
for i in range(n):
	if s1[i]!=s2[i]:
		diff+=1
if t<diff:
	if t*2>=diff:
		res=[None]*n
		singole=t*2-diff
		curr=1
		for i in range(n):
			if s1[i]==s2[i]:
				res[i]=s1[i]
			else: #diff
				if singole>0:
					res[i]=get_diff(s1[i],s2[i])
					singole-=1
				else:
					if curr==1:
						res[i]=s1[i]
					else:
						res[i]=s2[i]
					curr=-curr

elif t==diff:
	res=[None]*n
	for i in range(n):
		if s1[i]==s2[i]:
			res[i]=s1[i]
		else:
			res[i]=get_diff(s1[i],s2[i])
else: #t>diff
	res=[None]*n
	for i in range(n):
		if s1[i]==s2[i]:
			if t>diff:
				res[i]=get_diff(s1[i],s2[i])
				t-=1
			else:
				res[i]=s1[i]
		else:
			res[i]=get_diff(s1[i],s2[i])
res="".join(res)
print(res)