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 getDiffChar(c1, c2) :
    if c1 != 'a' and c2 != 'a' :
        return 'a'
    elif c1 != 'b' and c2 != 'b' :
        return 'b'
    else :
        return 'c'

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

diff = 0
for i in range(n) :
    if s1[i] != s2[i] :
        diff += 1
same = n - diff

if t < diff/2 :
    ans = -1
elif t < diff :
    s3_list = []
    p1 = p2 = diff - t
    for i in range(n) :
        if s1[i] == s2[i] :
            s3_list.append(s1[i])
        elif p1 :
            s3_list.append(s1[i])
            p1 -= 1
        elif p2 :
            s3_list.append(s2[i])
            p2 -= 1
        else :
            s3_list.append(getDiffChar(s1[i],s2[i]))
    s3 = "".join(s3_list)
    ans = s3
else :
    s3_list = []
    p = t - diff
    for i in range(n) :
        if s1[i] == s2[i] :
            if p :
                s3_list.append(getDiffChar(s1[i], s2[i]))
                p -=1
            else :
                s3_list.append(s1[i])
        else :
            s3_list.append(getDiffChar(s1[i],s2[i]))
    s3 = "".join(s3_list)
    ans = s3
print(ans)