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:
def no(s1,s2):
    if s1 != 'a' and s2 != 'a':
        return 'a'
    elif s1 != 'b' and s2 != 'b':
        return 'b'
    return 'c'

n,t = map(int,raw_input().split())

s  = [raw_input(),raw_input()]




same = 0

for s1,s2 in zip(s[0],s[1]):
    if s1 == s2:
        same += 1
        
diff = n - same

a = min(n-t,diff/2)
b = a
d = t - a
c = n - t - a


#print a,b,c,d,diff,same

if a + b > diff:
    print -1
elif c > same:
    print -1
elif a+b+c+d != n:
    print "!!!"
else:
    ans = ''
    for s1,s2 in zip(s[0],s[1]):
        if s1 != s2:
            if a != 0:
                ans += s1
                a -= 1
            elif b != 0:
                ans += s2
                b -= 1
            else:
                ans += no(s1,s2)
                d -= 1

        else:
            if c > 0:
                ans += s1
                c -= 1
            elif d > 0:
                ans += no(s1,s1)
                d -= 1
            
        
    print ans