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.*;

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

		String s1 = sc.next();
		String s2 = sc.next();

		int onaji = 0;
		int tigau = 0;

		for(int i = 0; i < n; i++){
			if(s1.charAt(i) == s2.charAt(i)){
				onaji++;
			}else{
				tigau++;
			}
		}

		if(tigau / 2 + onaji < same){
			System.out.println("-1");
		}else{
			int t1 = Math.min(same, tigau / 2);
			int t2 = t1;
			int o = same - t1;
			StringBuilder build = new StringBuilder();
			for(int i = 0; i < n; i++){
				if(s1.charAt(i) == s2.charAt(i)){
					if(o > 0){
						build.append(s1.charAt(i));
						o--;
					}else{
						build.append(s1.charAt(i) == 'a' ? 'b' : 'a');
					}
				}else{
					if(t1 > 0){
						build.append(s1.charAt(i));
						t1--;
					}else if(t2 > 0){
						build.append(s2.charAt(i));
						t2--;
					}else{
						if(s1.charAt(i) == 'a'){
							build.append(s2.charAt(i) == 'b' ? 'c' : 'b');
						}else if(s2.charAt(i) == 'a'){
							build.append(s1.charAt(i) == 'b' ? 'c' : 'b');
						}else{
							build.append('a');
						}
					}

				}
			}
			System.out.println(build.toString());
		}

	}
}