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.*;

public class c1 {
	public static void main(String[] args) throws FileNotFoundException {
		Scanner in = new Scanner(System.in);
		PrintWriter out = new PrintWriter(System.out);

		int n = in.nextInt();
		int t = in.nextInt();

		String s = in.next();
		String s1 = in.next();

		int ok = 0;
		
		t = n - t;
		
		for (int i = 0; i < n; i++) {
			if (s.charAt(i) == s1.charAt(i)) {
				ok++;
			}
		}

		if (t <= ok) {
			int cur = 0;

			for (int i = 0; i < n; i++) {
				if (s.charAt(i) == s1.charAt(i) && cur < t) {
					cur++;
					out.print(s.charAt(i));
				}
				else{
					for (int j = 0; j < 26; j++) {
						char c = (char) ((int) 'a' + j);
						
						if (c != s.charAt(i) && c != s1.charAt(i)){
							out.print(c);
							break;
						}
					}
				}
			}
		} else {
			if ((n - ok) / 2 >= (t - ok)){
				int count1 = 0;
				int count2 = 0;
				
				for (int i = 0; i < n; i++) {
					if (s.charAt(i) == s1.charAt(i)){
						out.print(s.charAt(i));
					}
					else{
						if (count1 < t - ok){
							out.print(s.charAt(i));
							count1++;
						}
						else{
							if (count2 < t - ok){
								out.print(s1.charAt(i));
								count2++;
							}
							else{
								for (int j = 0; j < 26; j++) {
									char c = (char) ((int) 'a' + j);
									
									if (c != s.charAt(i) && c != s1.charAt(i)){
										out.print(c);
										break;
									}
								}
							}
						}
					}
				}
			}
			else{
				out.println(-1);
			}
		}
		out.close();
	}
}