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:
II = lambda: int(raw_input())
IAI = lambda: map(int, raw_input().split())
IL = lambda: raw_input().strip()
IAS = lambda: raw_input().split()

n,t = IAI()
a = IL()
b = IL()

def sc(*x):
    if 'a' not in x: return 'a'
    if 'b' not in x: return 'b'
    return 'c'
dif = sum(map(str.__ne__,a,b))
lb = (dif+1)/2
if lb > t:
    print -1
else:
    D = min(dif,t)
    pa = pb = dif - D
    r = t - D
    o = ''
    for c,d in zip(a,b):
        if c==d:
            if r: r-=1; o += sc(c)
            else: o += c
        else:
            if pa: pa-=1; o += c
            elif pb: pb-=1; o += d
            else: o += sc(c,d)
    print o