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())
s = list(raw_input())
t = list(raw_input())
same = [0 for x in range(n)]
lets = 'abcdefghijklmnopqrstuvwxyz'
for i in range(n):
  if s[i]==t[i]:
    same[i] = 1
ans = ['' for x in range(n)]
cnt = 0
cnt2 = 0
if sum(same) >= n-k:
  cur = sum(same)-n+k
  for i in range(n):
    if same[i] and cur:
      cur -= 1
    elif same[i]:
      ans[i] = s[i]
      continue
    for j in range(26):
      if lets[j] != s[i] and lets[j] != t[i]:
        ans[i] = lets[j]
        break
else:
  for i in range(n):
    if same[i]:
      ans[i] = s[i]
      continue
    if cnt > cnt2:
      ans[i] = s[i]
      cnt2 += 1
    else:
      ans[i] = t[i]
      cnt += 1
  for i in range(n):
    if cnt < k and cnt2 < k and same[i]:
      for j in range(26):
        if lets[j] != s[i] and lets[j] != t[i]:
          ans[i] = lets[j]
          break
      cnt += 1
      cnt2 += 1
    elif cnt < k and ans[i]==s[i] and not same[i]:
      for j in range(26):
        if lets[j] != s[i] and lets[j] != t[i]:
          ans[i] = lets[j]
          break
      cnt += 1
    elif cnt2 < k and ans[i]==t[i] and not same[i]:
      for j in range(26):
        if lets[j] != s[i] and lets[j] != t[i]:
          ans[i] = lets[j]
          break
      cnt2 += 1
if cnt > k or cnt2 > k:
  print -1
else:
  print "".join(ans)