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,m=map(int, input().split())
a=input()
b=input()

did=[0]*(n+1)
for i in range(n):
    if a[i]==b[i]:
        did[i]=-1
k=m
for i in range(n):
    if k==0:
        break
    if not did[i]:
        did[i]=2
        k-=1
if k:
    for i in range(n):
        if k==0:
            break
        if did[i]==-1:
            did[i]=3
            k-=1
k=m
for i in range(n):
    if k==0:
        break
    if not did[i]:
        did[i]=1
        k-=1
if k:
    for i in range(n):
        if k==0:
            break
        if did[i]==2:
            did[i]=3
            k-=1
f=0
for i in range(n):
    if did[i]==0:
        f=1
        break
if f==0:
    for i in range(n):
        if did[i]==-1:
            print(a[i], end='')
        elif did[i]==1:
            print(b[i], end='')
        elif did[i]==2:
            print(a[i], end='')
        else:
            for j in range(26):
                if chr(ord('a')+j)!=a[i] and chr(ord('a')+j)!=b[i]:
                    print(chr(ord('a')+j),end='')
                    break
else:
    print(-1)