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:
n, t = map(int, input().split())
s1 = input()
s2 = input()
co = 0
for i in range(n):
    if s1[i] == s2[i]:
        co += 1
t = n-t
from_co = min(co, t)
from_s1, from_s2 = max(t-from_co, 0), max(t-from_co, 0)
if from_co + from_s1 + from_s2 > n:
    print(-1)
else:
    new_s = []
    for i in range(n):
        #print(co, from_s1, from_s2, s1[i], s2[i])
        if from_co > 0 and s1[i] == s2[i]:
            new_s.append(s1[i])
            from_co -= 1
        elif from_s1 > 0 and s1[i] != s2[i]:
            new_s.append(s1[i])
            from_s1 -= 1
        elif from_s2 > 0 and s1[i] != s2[i]:
            new_s.append(s2[i])
            from_s2 -= 1
        else:
            #print('hi')
            f = ['a','b','c']
            if s1[i] in f: f.remove(s1[i])
            if s2[i] in f: f.remove(s2[i])
            new_s.append(f[0])
    print(''.join(new_s))