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,stdout
nmbr = lambda: int(stdin.readline())
lst = lambda: list(map(int,stdin.readline().split()))
for _ in range(1):#nmbr()):
    n,k=lst()
    a=input()
    b=input()
    same=set()
    for i in range(n):
        if a[i]==b[i]:same.add(i)
    common=len(same)
    tobesame=n-k
    effec_same_needed=max(0,tobesame-common)
    effec_len=n-common
    # print(tobesame,effec_same_needed,effec_len)
    if effec_same_needed*2>effec_len:
        print(-1)
    else:
        ans=[' ']*n
        vis=set()
        for ind in same:
            if tobesame==0:break
            ans[ind]=a[ind]
            vis.add(ind)
            tobesame-=1
        c=effec_same_needed
        for i in range(n):
            if i in vis:continue
            if c==0:break
            ans[i]=a[i]
            vis.add(i)
            c-=1
        c=effec_same_needed
        for i in range(n):
            if i in vis: continue
            if c == 0: break
            ans[i] = b[i]
            vis.add(i)
            c -= 1
        for i in range(n):
            if i in vis:continue
            for asc in range(97,123):
                if chr(asc) not in [a[i],b[i]]:
                    ans[i]=chr(asc)
                    break
        print(''.join(ans))