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:
#!/usr/bin/env python

[n, t] = map(int, raw_input().split())

s1 = raw_input().strip()
s2 = raw_input().strip()

same = sum(1 if a==b else 0 for (a,b) in zip(s1,s2))

def different(a,b):
  if ("a" != a) and ("a" != b):
    return "a"
  if ("b" != a) and ("b" != b):
    return "b"
  if ("c" != a) and ("c" != b):
    return "c"

nr_to_copy_a = n-t
nr_to_copy_b = n-t
result = []
for (a,b) in zip(s1,s2):
  if (a == b) and (nr_to_copy_a > 0) and (nr_to_copy_b > 0):
    result.append(a)
    nr_to_copy_a -= 1
    nr_to_copy_b -= 1
  else:
    result.append(None)
result2 = []
for (a,b,c) in zip(s1,s2,result):
  if (c == None):
    if (nr_to_copy_a > 0):
      result2.append(a)
      nr_to_copy_a -= 1
    elif (nr_to_copy_b > 0):
      result2.append(b)
      nr_to_copy_b -= 1
    else:
      result2.append(different(a,b))
  else:
    result2.append(c)

if (nr_to_copy_a == 0) and (nr_to_copy_b == 0):
  print "".join(result2)
else:
  print -1