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())
s=set("qwertyuioplkjhgfdsazxcvbnm")
s1=input()
s2=input()
k1=k2=0
for i in range(n):
    if s1[i]==s2[i]: k1+=1
    else: k2+=1
if t<k2//2+k2%2: print(-1)
else: 
  t-=k2%2  
  k2//=2
  while t>k2 and k2>0: t-=2; k2-=1
  if k2==0 and t>k1: print(-1)
  else:     
    if k2==0: k1=t 
    else: k1=0
    k3=k2
    for i in range(n):
      if s1[i]==s2[i]: 
        ss=s-{s1[i]}
        if k1>0: print(ss.pop(),end='');k1-=1
        else: print(s1[i],end='')
      else:
        ss=s-{s1[i],s2[i]}
        if k3>k2: print(s1[i],end='');k3-=1
        elif k2>0: print(s2[i],end='');k2-=1
        else: print(ss.pop(),end='')