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:
#import sys
#input=sys.stdin.readline
l=input().split()
n=int(l[0])
k=int(l[1])
s1=input()
s2=input()
count=0
for i in range(n):
    if(s1[i]!=s2[i]):
        count+=1
if(count>2*k):
    print(-1)
    quit()
pp=0
for i in range(count+1):
    if((count-i)%2):
        continue
    done=i+((count-i+1)//2)
    if(k>=done and k-done<=n-count):
        samed=k-done
        sames=(n-count-samed)
        diffd=i
        diffs=(count-i)
        break
#print(count,diffd,diffs,sames,samed)
for i in range(n):
    if(s1[i]==s2[i]):
        if(samed):
            ok=(ord(s1[i])-ord('a'))+1
            ok%=26
            print(chr(ord('a')+ok),end="")
            samed-=1
        else:
            print(s1[i],end="")
            sames-=1
    else:
        if(diffd):
            p=['a','b','c']
            for j in p:
                if(j!=s1[i] and j!=s2[i]):
                    print(j,end="")
                    break
            diffd-=1
        else:
            if(diffs%2):
                print(s1[i],end="")
                diffs-=1
            else:
                print(s2[i],end="")
                diffs-=1
print()