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:

n,t=map(int,input().split())
s1=input()
s2=input()
same=n-t
poss=[]
left=[]
s=[0 for _ in range(n)]
for i in range(n):
    if s1[i]==s2[i]:
        poss.append(i)
    else:
        left.append(i)

if len(poss)>=same:
    for i in range(same):
        s[poss[i]]=s1[poss[i]]
    for i in range(n):
        if s[i]==0:
            for j in range(26):
                if chr(j+97)!=s1[i] and chr(j+97)!=s2[i]:
                    s[i]=chr(j+97)
                    break
    ans=''
    for i in s:
        ans+=i
    print(ans)
else:
    for i in range(min(same,len(poss))):
        s[poss[i]]=s1[poss[i]]
    rem=n-t-len(poss)
    for i in range(n):
        if s[i]==0:
            s[i]=s1[i]
            rem-=1
        if rem==0:
            break
    if rem!=0:
        print(-1)
        exit(0)
    rem = n - t - len(poss)

    for i in range(n):
        if s[i] == 0:
            s[i] = s2[i]
            rem -= 1
        if rem == 0:
            break
    if rem!=0:
        print(-1)
        exit(0)
    for i in range(n):
        if s[i]==0:
            for j in range(26):
                if chr(j+97)!=s1[i] and chr(j+97)!=s2[i]:
                    s[i]=chr(j+97)
                    break
    ans=''
    for i in s:
        ans+=i
    print(ans)