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 = list(map(int, input().split()))
s1, s2 = input(), input()

def f(s1, s2):
    ans = 0
    for i in range(len(s1)):
        if s1[i] != s2[i]:
            ans += 1
    return ans

def other(ch1, ch2):
    if ch1 != 'a' != ch2:
        return 'a'

    if ch1 != 'b' != ch2:
        return 'b'

    if ch1 != 'c' != ch2:
        return 'c'

if f(s1, s2) > t:
    ans = ''
    t1 = t2 = f(s1, s2) - t
    for i in range(n):
        if s1[i] == s2[i]:
            ans += s1[i]
            continue
        if t1 > 0:
            ans += s1[i]
            t1 -= 1
            continue
        if t2 > 0:
            ans += s2[i]
            t2 -= 1
            continue
        ans += other(s1[i], s2[i])
    if t1 != 0 or t2 != 0:
        print("-1")
        exit()
    print(ans)
    exit()

ans = ''
for i in range(n):
    if s1[i] != s2[i]:
        ans += other(s1[i], s2[i])
        continue
    ans += s1[i]

t -= f(s1, s2)

ans2 = ''
for i in range(n):
    if t > 0 and s1[i] == s2[i]:
        ans2 += other(s1[i], s2[i])
        t -= 1
        continue
    ans2 += ans[i]

print(ans2)