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 PYTHON):

Solution:
n,k=map(int,raw_input().split())
a=list(raw_input())
b=list(raw_input())
c=a[:]
d=[i for i in range(n) if a[i]!=b[i]]
while k and len(d)>k:
  i=d.pop()
  c[i]=a[i]
  i=d.pop()
  c[i]=b[i]
  k-=1
while d and k:
  i=d.pop()
  c[i]=[x for x in 'abc' if x!=a[i] and x!=b[i]][0]
  k-=1
if k:
  for i in range(n):
    if a[i]==b[i]:
      c[i]=[x for x in 'ab' if x!=a[i]][0]
      k-=1
      if not k:
        break
print -1 if k or d else ''.join(c)