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.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

/**
 * Created by pallavi on 6/10/15.
 */
public class B584 {
    static int n;
    static int t;
    public static char df(char a, char b) {
        int aa = a-97;
        int bb = b-97;
        int cc = (aa+1)%26;
        if (bb == cc) cc = (aa+2)%26;
        return (char) (cc+97);
    }
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        String[] inp = reader.readLine().split(" ");
        n = Integer.parseInt(inp[0]);
        t = Integer.parseInt(inp[1]);
        String s1 = reader.readLine();
        String s2 = reader.readLine();
        char[] res = new char[n];
        int diff = 0;
        for (int i = 0; i < n; i++) {
            if (s1.charAt(i) != s2.charAt(i)) diff+=1;
        }
        if (2*t<diff) {
            System.out.println(-1);
            return;
        }
        boolean fl = true;
        for (int i = 0; i < n; i++) {
            if (s1.charAt(i) == s2.charAt(i)) continue;
            if (2*t>diff) {
                res[i] = df(s1.charAt(i), s2.charAt(i));
                diff-=1;
                t-=1;
            } else {
                if (fl) {
                    res[i] = s1.charAt(i);
                    fl = false;
                } else {
                    res[i] = s2.charAt(i);
                    fl = true;
                    t-=1;
                    diff-=2;
                }
            }
        }
        for (int i = 0; i < n; i++) {
            if (s1.charAt(i) != s2.charAt(i)) continue;
            if (t>0) {
                res[i] = df(s1.charAt(i), s2.charAt(i));
                t-=1;
            } else {
                res[i] = s1.charAt(i);
            }
        }
        System.out.println(String.valueOf(res));
    }
}