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:
n,d=map(int,raw_input().split())
s1,s2=raw_input(),raw_input()
k=n-d
same=[s1[i]==s2[i] for i in range(n)].count(1)
if k-same<<1<=n-same:
    ans=[0]*n
    k1=k2=max(k-same,0)
    same=min(same,k)
    for i in range(n):
        if s1[i]!=s2[i]:
            if k1: k1-=1; ans[i]=s1[i]
            elif k2:k2-=1; ans[i]=s2[i]
            else: ans[i]=[x for x in 'abc' if x not in (s1[i],s2[i])][0]
        elif same:
            same-=1
            ans[i]=s1[i]
        else:
            ans[i]='a' if s1[i]!='a' else 'b'
    print ''.join(ans)
else: print -1