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.Scanner;

public class B{
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int n = scan.nextInt();
		int t = scan.nextInt();
		String s1 = scan.next();
		String s2 = scan.next();
		int diff = 0;
		int sim = 0;
		for(int i = 0; i < s1.length(); i++) {
			if(s1.charAt(i) != s2.charAt(i)) {
				diff ++;
			} else {
				sim ++;
			}
		}
		int diff1 = diff;
		int diff2 = diff;
		StringBuilder sb = new StringBuilder();
		for(int i = 0; i < s1.length(); i++) {
			if(s1.charAt(i) != s2.charAt(i)) {
				if(diff1 == diff2) {
					if(t * 2 > diff) {
						for(int j = 0; j < 26; j++) {
							char ch = (char) ('a' + j);
							if(ch != s1.charAt(i) && ch != s2.charAt(i)) {
								sb.append(ch);
								diff1 --;
								diff2 --;
								diff --;
								t--;
								break;
							}
						}
					} else {
						sb.append(s1.charAt(i));
						diff2 --;
						diff --;
					}
				} else if(diff1 < diff2) {
					sb.append(s1.charAt(i));
					t--;
					diff2 --;
					diff --;
				} else {
					sb.append(s2.charAt(i));
					t--;
					diff1 --;
					diff --;
				}
			} else {
				if(t > diff) {
					for(int j = 0; j < 26; j++) {
						char ch = (char) ('a' + j);
						if(ch != s1.charAt(i) && ch != s2.charAt(i)) {
							sb.append(ch);
							t--;
							break;
						}
					}
				} else {
					sb.append(s1.charAt(i));
				}
			}
			if(t * 2 < diff) {
				System.out.println(-1);
				return;
			}
		}
		if(t == 0 && diff1 == diff2) {
			System.out.println(sb.toString());
			return;
		}
		System.out.println(-1);

	}
}