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:
from random import choice

letters = [chr(letter) for letter in range(ord('a'),ord('z') + 1)]

def solve(n,t,s1,s2):
    same_char_count = n-t
    c = sum(char1 == char2 for char1,char2 in zip(s1,s2))
    if same_char_count <= c:
        s3 = list(s1)
        count = 0
        for i in range(len(s1)):
            if s1[i] == s2[i] and count < same_char_count:
                count += 1
                pass
            else:
                while True:
                    char = choice(letters)
                    if char != s1[i] and char != s2[i]:
                        s3[i] = char
                        break
        print(''.join(s3))
        return
    else:
        s4 = list(s1)
        count1,count2 = 0,0
        for i in range(len(s1)):
            if s1[i] != s2[i]:
                if count1 < count2 and count1 < same_char_count - c:
                    s4[i] = s1[i]
                    count1 += 1
                elif count2 <= count1 and count2 < same_char_count - c:
                    s4[i] = s2[i]
                    count2 += 1
                else:
                    while True:
                        char = choice(letters)
                        if char != s1[i] and char != s2[i]:
                            s4[i] = char
                            break
        c1 = sum(char1 == char2 for char1,char2 in zip(s1,s4))
        c2 = sum(char1 == char2 for char1,char2 in zip(s2,s4))
        if c1 == same_char_count and c2 == same_char_count:
            print(''.join(s4))
            return
    print(-1)

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

solve(n,t,s1,s2)