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 Main {
    public static void printDiff(char c1, char c2) {
        for (char c = 'a'; c <= 'z'; c ++) {
            if (c != c1 && c != c2) {
                System.out.print(c);
                break;
            }
        }
    }
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt(), t = sc.nextInt();
        sc.nextLine();
        String s1 = sc.nextLine(), s2 = sc.nextLine();

        int requiredMatch = 2 * (n - t);
        boolean[] diff = new boolean[n];
        for (int i = 0; i < n; i ++)
            diff[i] = s1.charAt(i) != s2.charAt(i);

        int freeHalf = 0;
        for (int i = 0; i < n; i ++)
            freeHalf += diff[i] ? 1 : 0;

        if (2 * n - freeHalf < requiredMatch) {
            System.out.println(-1);
            return;
        }

        int half1 = 0;
        int half2 = 0;

        for (int i = 0; i < n; i ++) {
            if (requiredMatch > 0) {
                if (diff[i]) {
                    if (half1 == half2 && freeHalf < 2) {
                        printDiff(s1.charAt(i), s2.charAt(i));
                    } else {
                        if (half1 <= half2) {
                            System.out.print(s1.charAt(i));
                            half1 ++;
                        } else {
                            System.out.print(s2.charAt(i));
                            half2 ++;
                        }
                        freeHalf --;
                        requiredMatch --;
                    }
                } else {
                    if (requiredMatch >= 2) {
                        System.out.print(s1.charAt(i));
                        requiredMatch -= 2;
                    } else {
                        printDiff(s1.charAt(i), s2.charAt(i));
                    }
                }
            } else {
                printDiff(s1.charAt(i), s2.charAt(i));
            }
        }
    }
}