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:
li= map(int, raw_input().split())
n = li[0]
t = li[1]
a = raw_input()
b = raw_input()

d = 0
rn = range(n)
for i in rn:
    if a[i] != b[i]:
        d += 1

c = list(a)
if t >= d:
    res = t-d
    for i in rn:
        if a[i] == b[i]:
            if res > 0:
                if a[i] == 'c':
                    c[i] = 'd'
                else:
                    c[i] ='c'
                res -= 1
            else:
                c[i] = a[i]
        else:
            if (a[i] != 'c' and b[i] != 'c'):
                c[i]='c'
            elif (a[i] != 'd' and b[i] != 'd'):
                c[i]='d'
            else :
                c[i]= 'e'

else:
    k = d - t
    k2 = 2*k
    if k2> d:
        print "-1"
        exit()
    flag = True
    for i in rn:
        #print i
        
        if a[i] == b[i]:
            c [i]= a[i]
        else:
            if (k2 == 0):
                if (a[i] != 'c' and b[i] != 'c'):
                    c[i]='c'
                elif (a[i] != 'd' and b[i] != 'd'):
                    c[i]='d'
                else :
                    c[i]= 'e'
            else:
                k2 -= 1
                if flag :
                    c[i]=a[i]
                else :
                    c[i]= b[i]
                flag = not flag

cc = "".join(c)
print cc