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 JAVA):

Solution:

import java.util.Scanner;

/*
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 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */

/**
 *
 * @author workzone
 */
public class different_string_k {
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        int n,t;
        n=sc.nextInt();
        t=sc.nextInt();
        sc.nextLine();
        String str1,str2;
        str1=sc.nextLine();
        str2=sc.nextLine();
        char str3[]=new char[100005];
        int same=n-t;
        int tempsame=same;
        for(int i=0;i<n;++i)
        {
            str3[i]='0';
        }
        for(int i=0;i<n;++i)
        {
            if(same==0)
            {
                break;
            }
            else
            {
                if(str1.charAt(i)==str2.charAt(i))
                {
                    same--;
                    str3[i]=str1.charAt(i);
                }
            }
        }
        for(int i=0;i<n;++i)
        {
            if(str3[i]=='0')
            {
                str3[i]='a';
                while(str3[i]==str1.charAt(i) || str3[i]==str2.charAt(i))
                {
                    str3[i]++;
                }
            }
        }
        if(same>0)
        {
            int same1=same;
            int same2=same;
            for(int i=0;i<n;++i)
            {
                if(same1>0 && str1.charAt(i)!=str2.charAt(i))
                {
                    same1--;
                    str3[i]=str1.charAt(i);
                }
                else if(same2>0 && str1.charAt(i)!=str2.charAt(i))
                {
                    same2--;
                    str3[i]=str2.charAt(i);
                }
            }
        }
        int same1=0;
        int same2=0;
        for(int i=0;i<n;++i)
        {
            if(str3[i]==str1.charAt(i))
            {
                same1++;
            }
            if(str3[i]==str2.charAt(i))
            {
                same2++;
            }
        }
        if(same1==tempsame && same2==tempsame)
        {
            for(int i=0;i<n;++i)
            {
                System.out.print(str3[i]);
            }
        }
        else
        {
            //System.out.println(str3);
            System.out.println(-1);
        }
    }
}