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:
#------------------------template--------------------------#
import os
import sys
from math import *
from collections import *
from fractions import *
from bisect import *
from heapq import*
from io import BytesIO, IOBase
def vsInput():
    sys.stdin = open('input.txt', 'r')
    sys.stdout = open('output.txt', 'w')
BUFSIZE = 8192
class FastIO(IOBase):
    newlines = 0
    def __init__(self, file):
        self._fd = file.fileno()
        self.buffer = BytesIO()
        self.writable = "x" in file.mode or "r" not in file.mode
        self.write = self.buffer.write if self.writable else None
    def read(self):
        while True:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            if not b:
                break
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines = 0
        return self.buffer.read()
    def readline(self):
        while self.newlines == 0:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            self.newlines = b.count(b"\n") + (not b)
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines -= 1
        return self.buffer.readline()
    def flush(self):
        if self.writable:
            os.write(self._fd, self.buffer.getvalue())
            self.buffer.truncate(0), self.buffer.seek(0)
class IOWrapper(IOBase):
    def __init__(self, file):
        self.buffer = FastIO(file)
        self.flush = self.buffer.flush
        self.writable = self.buffer.writable
        self.write = lambda s: self.buffer.write(s.encode("ascii"))
        self.read = lambda: self.buffer.read().decode("ascii")
        self.readline = lambda: self.buffer.readline().decode("ascii")
sys.stdin, sys.stdout = IOWrapper(sys.stdin), IOWrapper(sys.stdout)
input = lambda: sys.stdin.readline().rstrip("\r\n")
def value():return tuple(map(int,input().split()))
def array():return [int(i) for i in input().split()]
def Int():return int(input())
def Str():return input()
def arrayS():return [i for i in input().split()]

#-------------------------code---------------------------#
#vsInput()

n,t=value()
s1=input()
s2=input()
s3=[0]*(n)
ot=t

dif=[]
equal=[]

for i in range(n): 
    if(s1[i]!=s2[i]): dif.append(i)
    else: equal.append(i)


i=0
while(i+1<len(dif)):
    s3[dif[i]]=1
    s3[dif[i+1]]=2
    t-=1
    i+=2
if(len(dif)%2):
    s3[dif[-1]]=3
    t-=1

i=0
while(t>0 and i+1<len(dif)):
    t-=1
    s3[dif[i]]=3
    s3[dif[i+1]]=3
    i+=2

i=0
while(t>0 and i<len(equal)):
    s3[equal[i]]=3
    t-=1
    i+=1


d1=0
d2=0

for i in range(n):
    if(s1[i]==1): s3[i]=s1[i]
    elif(s3[i]==2): s3[i]=s2[i]
    elif(s3[i]==3):
        for c in 'abc':
            if(s1[i]!=c and s2[i]!=c):
                s3[i]=c
                break
    else:
        s3[i]=s1[i]
    d1+=s1[i]!=s3[i]
    d2+=s2[i]!=s3[i]

#print(s3,d1,d2)

if(d1==d2 and d1==ot): print(*s3,sep="")
else: print(-1)