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=str(input())
s2=str(input())
ans=[]
mode=1
t=n-t
last=-1
for i in range(n):
    if t==0:
        if s1[i]!="a" and s2[i]!="a":
            ans.append("a")
        elif s1[i]!="b" and s2[i]!="b":
            ans.append("b")
        else:
            ans.append("c")
    elif s1[i]==s2[i]:
        t-=1
        ans.append(s1[i])
    elif mode==1:
        ans.append(s1[i])
        mode=0
        last=i
    else:
        ans.append(s2[i])
        mode=1
        t-=1
        last=-1
if last>=0:
    if s1[last]!="a" and s2[last]!="a":
        ans[last]="a"
    elif s1[last]!="b" and s2[last]!="b":
        ans[last]="b"
    else: ans[last]="c"
if t==0:
    for i in ans:
        print(i,end="")
else:
    print(-1)