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 PYTHON):

Solution:
n,t=map(int,raw_input().split())
st1=str(raw_input())
st2=str(raw_input())
c=0
ans=[0]*(n+2)
def cal(a,b):
    if a!="a" and b!="a":
        return "a"
    if a!="b" and b!="b":
        return "b"
    if a!="c" and b!="c":
        return "c"
for i in range(0,n):
    if st1[i]!=st2[i]:
        c=c+1
if 1==2:
    print -1
else:
    ans=[]
    c1=0
    for i in st2:
        ans.append(i)
    for i in range(0,n):
        if ans[i]!=st1[i]:
            c1=c1+1
    t1=c1
    t2=0
    l=0
    for i in range(0,n):
        if st1[i]==st2[i]:
            if t1<t and t2<t :
                t1=t1+1
                t2=t2+1
                ans[i]=cal(st1[i],st2[i])
        else:
            l=l+1
    vl=2*l-(t1-t2)
    #print t1,t2
    if t1>=t and t2<t:

        for i in range(0,n):
            if st1[i]!=st2[i]:
                if t1 > t and t2 < t:
                    ans[i]=st1[i]
                    t1=t1-1
                    t2=t2+1
                    #print ans[i],i
                elif t1<=t and t2<t:
                    ans[i]=cal(st1[i],st2[i])
                    t1=t1
                    t2 = t2+1
                #elif t1==t and t2<t:
                    #ans[i]=

            #print t1,t2
    if t1==t and t2==t:
        print "".join(ans)

    else:
        print -1