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.util.Scanner;

public class Three {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int t = sc.nextInt();

		int same = n - t;
		int blanks = n;

		char a[] = sc.next().toCharArray();
		char b[] = sc.next().toCharArray();
		char c[] = new char[n];

		for(int i = 0; i < n && same != 0; i++) {
			if(a[i] == b[i]) {
				c[i] = a[i];
				same--;
				blanks--;
			}
		}

		if(blanks / 2 < same) {
			System.out.println(-1);
			return;
		}

		int asame = same;
		int bsame = same;
		int ccount = -1;

		while(asame != 0 || bsame != 0) {
			ccount++;
			if(c[ccount] != 0)
				continue;

			if(bsame > asame) {
				c[ccount] = b[ccount];
				bsame--;
			}
			else {
				c[ccount] = a[ccount];
				asame--;
			}
		}

		while(ccount < n - 1) {
			ccount++;
			if(c[ccount] != 0)
				continue;
			c[ccount] = diff(a[ccount], b[ccount]);
		}

		System.out.println(String.valueOf(c));
	}

	public static char diff(char a, char b) {
		for(char i = 'a'; i <= 'z'; i++)
			if(a != i && b != i) return i;
		return '0';
	}
}