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:
/**
 * Created by User on 07.10.2015.
 */
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.Scanner;

public class FirstApp {
    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);

        int n, t;
        String s1, s2;

        n = new Integer(scanner.next());
        t = new Integer(scanner.next());
        int k = n - t;

        s1 = scanner.next();
        s2 = scanner.next();

        int q = 0;


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

        StringBuilder sb = new StringBuilder();
        char[] arr = {'a', 'b', 'c'};
        if (k <= q) {
            int c = k;
            for (int i = 0; i < n; i++) {
                if (s1.charAt(i) == s2.charAt(i) && c > 0) {
                    sb.append(s1.charAt(i));
                    c--;
                } else {
                    for (int j = 0; j < 3; j++) {
                        if ((arr[j] != s1.charAt(i)) && (arr[j] != s2.charAt(i))) {
                            sb.append(arr[j]);
                            break;
                        }
                    }

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

        else {
            if ((n - q) < 2 * (k - q)) {
                System.out.println(-1);
            }
            else {
                int c = k;
                int c2 = k - q;
                int z = -(k - q);
                for (int i = 0; i < n; i++) {
                    if (s1.charAt(i) == s2.charAt(i) && c > 0) {
                        sb.append(s1.charAt(i));
                        c--;
                    } else if (c2 > 0) {
                        sb.append(s1.charAt(i));
                        c2--;
                    } else if (c2 > z) {
                        sb.append(s2.charAt(i));
                        c2--;
                    } else {
                        for (int j = 0; j < 3; j++) {
                            if ((arr[j] != s1.charAt(i)) && (arr[j] != s2.charAt(i))) {
                                sb.append(arr[j]);
                                break;
                            }
                        }
                    }
                }
                System.out.println(sb.toString());
            }
        }

    }
}