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.ArrayList;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.BufferedReader;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.IOException;
import java.util.StringTokenizer;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
	public static void main(String[] args) {
		InputStream inputStream = System.in;
		OutputStream outputStream = System.out;
		Reader in = new Reader(inputStream);
		PrintWriter out = new PrintWriter(outputStream);
		TaskC solver = new TaskC();
		solver.solve(1, in, out);
		out.close();
	}
}

class TaskC {
    char other(char a, char b) {
        for (char c = 'a'; c < 'z'; c++) {
            if (c != a && c != b) return c;
        }
        return 'z';
    }
    public void solve(int testNumber, Reader in, PrintWriter out) {
        int n = in.nextInt();
        int t = in.nextInt();
        char[] s1 = in.next().toCharArray();
        char[] s2 = in.next().toCharArray();
        int diff = 0;
        for (int i = 0; i < n; i++) diff += (s1[i] != s2[i] ? 1 : 0);
        if (diff > 2 * t) {
            out.println(-1);
        } else {
            int a00 = Math.max((2 * t - diff + 1) / 2, 2 * t - n);
            int a01 = t - a00;
            int a10 = t - a00;
            int a11 = n - a01 - a10 - a00;
            if (a11 < 0) {
                out.println(-1);
            } else {
                StringBuffer ans = new StringBuffer();
                for (int i = 0; i < n; i++) {
                    if (s1[i] == s2[i]) {
                        if (a11 > 0) {
                            ans.append(s1[i]);
                            a11--;
                        } else {
                            ans.append(other(s1[i], s2[i]));
                            a00--;
                        }
                    } else {
                        if (a10 > 0) {
                            ans.append(s1[i]);
                            a10--;
                        } else if (a01 > 0) {
                            ans.append(s2[i]);
                            a01--;
                        } else {
                            ans.append(other(s1[i], s2[i]));
                            a00--;
                        }
                    }
                }
                if (a00 != 0 || a10 != 0 || a01 != 0 || a11 != 0) {
                    while (true) { }
                }
                out.println(ans);
            }
        }
    }
}

class Reader {
    private BufferedReader in;
    private StringTokenizer st;
    public Reader(InputStream is) {
        in = new BufferedReader(new InputStreamReader(is));
    }
    public String next() {
        try {
            while (st == null || !st.hasMoreTokens()) {
                st = new StringTokenizer(in.readLine());
            }
            return st.nextToken();
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    public int nextInt() {
        return Integer.parseInt(next());
    }

}