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:
n, t = map(int, input().split())
aa = list(input().strip())
bb = list(input().strip())
cc = n * [ ' ' ]

want_same = n - t
mismatched = 0

for i in range(n):
  if aa[i] == bb[i]:
    if want_same == 0:
      for c in ['y', 'z']:
        if c != aa[i]:
          cc[i] = c
    else:
      cc[i] = aa[i]
      want_same -= 1
  else:
    mismatched += 1
    for c in ['x', 'y', 'z']:
      if c != aa[i] and c != bb[i]:
        cc[i] = c
        break

if 2 * want_same > mismatched:
  print(-1)
  import sys; sys.exit()

if want_same != 0:
  want_same *= 2
  for i in range(n):
    if aa[i] != bb[i]:
      if want_same % 2 == 0:
        cc[i] = aa[i]
      else:
        cc[i] = bb[i]
      want_same -= 1
      if want_same == 0:
        break

print(''.join(cc))