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 string

n, t = map(int, input().split())
s1 = input()
s2 = input()

def otherchr(*args):
    for i in string.ascii_lowercase:
        if i not in args:
            return i
diff = []
same = []
str = [ch for ch in s1]
for i in range(n):
    if s1[i] != s2[i]:
        diff.append(i)
    else:
        same.append(i)
temp = len(diff)
if temp > 2 * t:
    print(-1)
else:
    if t > temp:
        count = t - temp
        for i in diff:
            str[i] = otherchr(s1[i], s2[i])
        for i in same[:count]:
            str[i] = otherchr(s1[i], s2[i])
            
    elif t == temp:
        for i in diff:
            str[i] = otherchr(s1[i], s2[i])
    else:
        count = temp - t
        for i in range(count, 2 * count):
            str[diff[i]] = s2[diff[i]]
        for i in range(2 * count, temp):
            str[diff[i]] = otherchr(s1[diff[i]], s2[diff[i]])               
    print(''.join(str))