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:
// practice with rainboy
import java.io.*;
import java.util.*;

public class CF584C extends PrintWriter {
	CF584C() { super(System.out, true); }
	Scanner sc = new Scanner(System.in);
	public static void main(String[] $) {
		CF584C o = new CF584C(); o.main(); o.flush();
	}

	byte get(byte a, byte b) {
		for (byte c = 'a'; ; c++)
			if (c != a && c != b)
				return c;
	}
	void main() {
		int n = sc.nextInt();
		int t = sc.nextInt();
		byte[] aa = sc.next().getBytes();
		byte[] bb = sc.next().getBytes();
		int d = 0;
		for (int i = 0; i < n; i++)
			if (aa[i] != bb[i])
				d++;
		if (d > t * 2) {
			println(-1);
			return;
		}
		byte[] cc = new byte[n];
		if (d >= t) {
			int t_ = t * 2 - d;
			int ta = t - t_;
			int tb = t - t_;
			for (int i = 0; i < n; i++)
				if (aa[i] != bb[i]) {
					if (t_ > 0) {
						cc[i] = get(aa[i], bb[i]);
						t_--;
					} else if (ta > 0) {
						cc[i] = bb[i];
						ta--;
					} else {
						cc[i] = aa[i];
						tb--;
					}
				} else
					cc[i] = aa[i];
		} else {
			int t_ = t - d;
			for (int i = 0; i < n; i++)
				if (aa[i] != bb[i])
					cc[i] = get(aa[i], bb[i]);
				else {
					if (t_ > 0) {
						cc[i] = get(aa[i], bb[i]);
						t_--;
					} else
						cc[i] = aa[i];
				}
		}
		println(new String(cc));
	}
}