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 sys
n, t = map(int, sys.stdin.readline().split())
ans = ["*" for _ in range(n)]
all = "abcdefghijklmnopqrstuvwxyz"
strings = tuple([sys.stdin.readline().replace("\n",""), sys.stdin.readline().replace("\n","")])
s = n - t
for i in range(n):
    if s != 0 and strings[0][i] == strings[1][i]:
        ans[i] = strings[0][i]
        s -= 1  
    if s == 0:
        break
turn = 0
s = s*2
t = t*2
for i in range(n):
    if s != 0 and ans[i] == "*":
        if strings[0][i] != strings[1][i]:
            ans[i] = strings[turn][i]
        else:
            ans[i] = all.replace(strings[turn][i], "")[0]
        turn = (turn + 1) % 2
        t -= 1
        s -= 1
    if s == 0 and ans[i] == "*":
        new = all.replace(strings[0][i], "")
        new = new.replace(strings[1][i], "")
        ans[i] = new[0]
        t -= 2
if s == 0 and t == 0:
    print("".join(ans))
else:
    print(-1)