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().rstrip()
S2 = input().rstrip()

diff = []
for i in range(N):
    if S1[i] != S2[i]:
        diff.append(i)

import sys

ans = [None for i in range(N)]
while len(diff) > T and T:
    for i in range(2):
        idx = diff[-1]
        diff.pop()
        if i == 1:
            ans[idx] = S1[idx]
        else:
            ans[idx] = S2[idx]
    T -= 1

if diff and not T:
    print(-1)
    sys.exit(0)

while T and diff:
    idx = diff[-1]
    diff.pop()
    for x in ('a', 'b', 'c'):
        if x == S1[idx] or x == S2[idx]:
            continue
        ans[idx] = x
        break
    T -= 1

assert(not diff)

for i in range(N):
    if not ans[i]:
        if T:
            T -= 1
            ans[i] = 'a' if S1[i] != 'a' else 'b'
        else:
            ans[i] = S1[i]

print(''.join(ans))