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.Arrays;
import java.util.Scanner;


public class CF324C {

	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();

		char c = 'a';
		char[] arr = new char[n];
		Arrays.fill(arr, c);

		int sameCount = 0;
		for(int i = 0; i < n; i++){
			if(s1.charAt(i) == s2.charAt(i)){
				sameCount++;
				arr[i] = s1.charAt(i);

			}
			else{
				if(s1.charAt(i) == 'a'|| s2.charAt(i) == 'a'){
					arr[i] = 'b';
					if (s1.charAt(i) == 'b'|| s2.charAt(i) == 'b'){
						arr[i] = 'c';
					}
				}

			}

		}

		if(2*n-2*t-sameCount > n){
			System.out.println("-1");
			return;
		}

		int shouldBeSame = n-t;
		int diff = shouldBeSame - sameCount;
		int diff2 = diff;

		int ind = 0;
		while(diff < 0 && ind<n){
			if (s1.charAt(ind) == s2.charAt(ind)){
				arr[ind] = arr[ind] == 'z'? 'a' : (char)(arr[ind]+1);
				diff++;
				ind++;
			}
			else{
				ind++;
			}

		}
		while(diff > 0 && ind<n){
			if (s1.charAt(ind) != s2.charAt(ind)){
				arr[ind] = s1.charAt(ind);
				diff--;
				ind++;
			}
			else{
				ind++;
			}
		}
		while(diff2 > 0 && ind<n){
			if (s1.charAt(ind) != s2.charAt(ind)){
				arr[ind] = s2.charAt(ind);
				diff2--;
				ind++;
			}
			else
				ind++;
		}

		String result = new String(arr);
		System.out.println(result);
	}

}