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:
def diffchar(a,b):
    for i in range(26):
        c=chr(ord('a')+i)
        if c!=a and c!=b:
            return c
while True:
    try:
        n,t=map(int,input().split())
        a,b=(input() for _ in range(2))
    except: raise SystemExit
    same=[i for i in range(n) if a[i]==b[i]]
    diff=[i for i in range(n) if a[i]!=b[i]]
    if 2*t<len(diff):
        print(-1)
        continue
    scnt,dcnt=n-t,t
    res=['.']*n
    for si in same:
        if scnt==0:
            break
        res[si]=a[si]
        scnt-=1
    if scnt>0:
        for i,di in enumerate(diff):
            if scnt==0:
                break
            res[di]=a[di] if i%2==0 else b[di]
            scnt-=0.5
    for i in range(n):
        if res[i]=='.':
            res[i]=diffchar(a[i],b[i])
    print("".join(res))