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:
def diff(c1, c2):
    alph = 'abcdefghijklmnopqrstuvwxyz'
    for c in alph:
        if c != c1 and c!= c2: return c

n, t = map(int, input().split())
s1 = input()
s2 = input()
s3 = []
same1 = 0
same2 = 0
last1 = -1
last2 = -1
i = 0
first = True
while (same1 < (n - t) or same2 < (n - t)) and i < n:
    if s1[i] == s2[i]:
        if same1 + 1 > (n-t):
            s3[last1] = diff(s1[last1], s2[last1])
            same1 -= 1
        elif same2 + 1 > (n-t):
            s3[last2] = diff(s1[last2], s2[last2])
            same2 -= 1
        s3.append(s1[i])
        same1 += 1
        same2 += 1
    elif s1[i] == s2[i]:
        s3 += diff(s1[i], s2[i])
    else:
        if first:
            s3.append(s1[i])
            same1 += 1
            last1 = i
        else:
            s3.append(s2[i])
            same2 += 1
            last2 = i
        first = not first
    i += 1

if same1 != same2 or (same1 == same2 != (n-t)):
    print(-1)
else:
    remaining = n - len(s3)
    for i in range(len(s3), n):
        s3 += diff(s1[i], s2[i])
    print(''.join(s3))