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.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.*;

/**
 * Created with IntelliJ IDEA.
 * User: Daniil
 * Date: 9/22/15
 * Time: 10:01 AM
 * To change this template use File | Settings | File Templates.
 */
public class Main {

    public static char[] res;

    public static void main(String[] args){

        Scanner scanner= new Scanner(new InputStreamReader(System.in));
        int n  = 0, t;


        n = scanner.nextInt();
        t = scanner.nextInt();
        scanner.nextLine();
        String f = scanner.nextLine();
        String s = scanner.nextLine();
        res = new char[n];

        Arrays.fill(res, ' ');
        int cnt = n - t;

        for (int i = 0;i < n ;++ i){
            if (cnt > 0 && f.charAt(i) == s.charAt(i)){
                cnt --;
                res[i] = f.charAt(i);
            }
        }

        int ft = cnt;
        int st = cnt;
        for (int i = 0;i < n ;++ i){
            if (res[i] == ' '){
                if (ft >= st && ft > 0){
                    res[i] = f.charAt(i);
                    ft --;
                }else if (st > 0 && st >ft){
                    res[i] = s.charAt(i);
                    st --;
                }
            }
        }
        if (ft != 0 || st != 0){
            System.out.println("-1");
        }else{
            for (int i = 0;i < n ;++ i){
                if (res[i] == ' '){
                    for (int j = 0; j < 26; ++ j){
                        if (s.charAt(i) != ('a' + j) && f.charAt(i) != ('a' + j)){
                            res[i] = (char)('a' + j);
                            break;
                        }
                    }
                }
            }
            System.out.println(res);
        }

    }




}