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.*;
import java.util.*;
import java.math.*;

public class Div2_324_B {
    public static void main(String[] args) throws IOException {
        BufferedInputStream bis = new BufferedInputStream(System.in);
        BufferedReader br = new BufferedReader(new InputStreamReader(bis));
        StringTokenizer st = new StringTokenizer(br.readLine());
        int n = Integer.parseInt(st.nextToken());
        int t = Integer.parseInt(st.nextToken());
        int same = 0, diff = 0;
        String s1 = br.readLine().trim();
        String s2 = br.readLine().trim();
        char [] res = new char [n];
        for (int i = 0; i < s1.length(); i++) {
            if(s1.charAt(i) != s2.charAt(i))
                diff++;
            else
                same++;
        }
        int cnt = 0, part = diff/2;
        for (int i = 0; i < res.length; i++) {
            if(s1.charAt(i) != s2.charAt(i)) {
                res[i] = getDiff(s1.charAt(i), s2.charAt(i));
                t--;
            } else
                res [i] = s1.charAt(i);
        }
        for (int i = 0; i < res.length && t < 0; i++) {
            if(s1.charAt(i) != s2.charAt(i)) {
                if(cnt%2 == 0) {
                    res[i] = s1.charAt(i);
                } else {
                    res[i] = s2.charAt(i);
                    t++;
                }
                cnt++;
            }
        }
        for(int i = 0;i < res.length && t > 0;i++) {
            if (s1.charAt(i) == s2.charAt(i)) {
                t--;
                if(s1.charAt(i) == 'z')
                    res[i] = 'a';
                else
                    res[i] = (char)(s1.charAt(i)+1);
            }
        }
        if(t == 0)
            System.out.println(new String(res));
        else
            System.out.println(-1);
    }
    public static char getDiff(char a, char b) {
        if(a > b) {
            char t = a;
            a = b;
            b = t;
        }
        if(b-a > 1)
            return (char)(a+1);
        if(b < 'z')
            return 'z';
        return 'a';
    }
}