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:
from sys import stdin
inp = lambda : stdin.readline().strip()

n, t = [int(x) for x in inp().split()]
a = inp()
b = inp()
ans=list(a[:])
diff=[i for i in range(n) if a[i]!=b[i]]
while t:
    if len(diff) <= t:
        break
    i=diff.pop()
    ans[i]=a[i]
    i=diff.pop()
    ans[i]=b[i]
    t-=1
while len(diff) > 0 and t > 0:
  i=diff.pop()
  ans[i]=[x for x in 'abc' if x!=a[i] and x!=b[i]][0]
  t-=1
if t:
  for i in range(n):
    if a[i]==b[i]:
      ans[i]=[x for x in 'ab' if x!=a[i]][0]
      t-=1
      if not t:
        break
if t > 0 or len(diff)> 0:
    print(-1)
else:
    print(''.join(ans))