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:
import sys
import time
t1=time.time()
from string import ascii_lowercase as lc
n,t=map(int,input().split())
#s1=sys.stdin.readline()
#s2=sys.stdin.readline()
s1=input()
s2=input()
ctr=0
for i in range(n):
    if s1[i]!=s2[i]:
        ctr+=1
ctr1,ctr2=ctr-t,ctr-t
if t>=ctr/2:    
    ans=[]
    if t>ctr:
        for i in range(n):
            if s1[i]==s2[i]:
                if t>ctr:
                    if s1[i]=='a':
                        ans.append('b')
                    else:
                        ans.append('a')
                    t-=1
                else:
                    ans.append(s1[i])
            else:
                for j in range(3):
                    if lc[j]!=s1[i] and lc[j]!=s2[i]:        
                        ans.append(lc[j])
                        break
                
    else:
        for i in range(n):
            if s1[i]==s2[i]:
                ans.append(s1[i])
            else:
                if ctr1!=0:
                    ans.append(s1[i])
                    ctr1-=1
                elif ctr2!=0:
                    ans.append(s2[i])
                    ctr2-=1
                else:
                    for j in range(3):
                        if lc[j]!=s1[i] and lc[j]!=s2[i]:        
                            ans.append(lc[j])
                            break
   
    print(''.join(a1 for a1 in ans))
    #sys.stdout.write(ans)
else:
    print(-1)