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()
diff=0
same=0
for i in range(len(s1)):
    if(s1[i]!=s2[i]):
        diff+=1
    else:
        same+=1
if(diff>2*t):
    print(-1)
    exit()
ans=['' for i in range(n)]
count=t
if(diff<=t):
    for i in range(len(s1)):
        if(s1[i]!=s2[i]):
            if(s1[i]!='a' and s2[i]!='a'):
                ans[i]='a'
            elif(s1[i]!='b' and s2[i]!='b'):
                ans[i]='b'
            elif(s1[i]!='c' and s2[i]!='c'):
                ans[i]='c'
            count-=1
        else:
            ans[i]=-1
    for i in range(len(s1)):
        if(s1[i]==s2[i]):
            if(count>0):
                if(s1[i]!='a' and s2[i]!='a'):
                    ans[i]='a'
                else:
                    ans[i]='b'
                count-=1
            else:
                ans[i]=s1[i]
            
    print(''.join(map(str,ans)))
else:
    ans=['' for i in range(len(s1))]
    c1=diff-t
    c2=diff-t
    for i in range(len(s1)):
        if(s1[i]!=s2[i]):
            if(c1):
                ans[i]=s1[i]
                c1-=1
            elif(c2):
                ans[i]=s2[i]
                c2-=1
            else:
                if(s1[i]!='a' and s2[i]!='a'):
                    ans[i]='a'
                elif(s1[i]!='b' and s2[i]!='b'):
                    ans[i]='b'
                elif(s1[i]!='c' and s2[i]!='c'):
                    ans[i]='c'
        else:
            ans[i]=s1[i]
    print(''.join(map(str,ans)))