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:
n, t = [int(x) for x in raw_input().split()]
a = raw_input()
b = raw_input()

def notthese(a, b):
    if a != 'a' and b != 'a':
        return 'a'
    else:
        if a != 'b' and b != 'b':
            return 'b'
        else:
            return 'c'


same = 0
diff = 0

for i in xrange(n):
    if a[i] == b[i]:
        same += 1
    else:
        diff += 1

ax = n - t - same
bx = ax
c = []

for i in xrange(n):
    if a[i] == b[i]:
        if ax < 0:
            c.append(notthese(a[i], b[i]))
            ax += 1
            bx += 1
        else:
            c.append(a[i])
    else:
        if ax > 0:
            c.append(a[i])
            ax -= 1
        elif bx > 0:
            c.append(b[i])
            bx -= 1
        else:
            c.append(notthese(a[i], b[i]))

if ax > 0 or bx > 0:
    print -1
else:
    print "".join(c)