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 PYTHON):

Solution:
in_data = raw_input().split()
n,t = int(in_data[0]), int(in_data[1])
s1 = raw_input()
s2 = raw_input()
a = 0
for i in range(n):
    if s1[i] == s2[i]:
        a += 1
if 2*t < n -a:
    print "-1"
else:
    count0, count1, count2 = 0, 0, 0
    k0 = min(n-t,a)
    k1 = n-t-k0
    k2 = k1
    output = []
    i = 0
    while i < n:
        if s1[i] == s2[i]:
            if count0 < k0:
                output.append(s1[i])
                count0 += 1
            else:
                if s1[i] == 'a':
                    output.append('b')
                else:
                    output.append('a')
        else:
            if count1 < k1:
                output.append(s1[i])
                count1 += 1
            elif count2 < k2:
                output.append(s2[i])
                count2 += 1
            else:
                if s1[i] == 'a':
                    if s2[i] == 'b':
                        output.append('c')
                    else:
                        output.append('b')
                elif s1[i] == 'b':
                    if s2[i] == 'a':
                        output.append('c')
                    else:
                        output.append('a')
                else:
                    if s2[i] == 'a':
                        output.append('b')
                    else:
                        output.append('a')
        i += 1
    answer = ''
    for entry in output:
        answer += entry
    print answer