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:
alp =["z","y","x","w","v","u","t","s","r","q","p","o","n","m","l","k","j","i","h","g","f","e","d","c","b","a"]


a,b = input().split()

a = int(a)
b = int(b)

s1 = input()
s2 = input()
if b == 0 and s1 != s2:
    print (-1)
else:
    c = 0
    d = []
    for i in range(a):
        if s1[i] == s2[i]:
            c = c + 1
    x = a-b        
    res = ""
    f = 0
    if c >= x:
        for i in range(a):
            if s1[i] == s2[i] and f != x:
                res += s1[i]
                f += 1
            else:
                for j in alp:
                    if j != s1[i] and j!= s2[i]:
                        res += j
                        break
 
    else:
        if a-c < 2*(x-c):
            print (-1)
        else:
            t = x-c
            q = 0
            for i in range(a):
                if s1[i] == s2[i] and f != x:
                    res += s1[i]
                elif q<t:
                    res += s1[i]
                    q += 1
                elif q<2*t:
                    res += s2[i]
                    q += 1
                else:
                    for j in alp:
                        if j != s1[i] and j!= s2[i]:
                            res += j
                            break

    print (res)