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.FileNotFoundException;
import java.io.InputStreamReader;
import java.io.IOException;
import java.io.FileReader;
import java.io.BufferedReader;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.StringTokenizer;
import java.math.BigInteger;
import java.io.InputStream;

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

class TaskC {
    public void solve(int testNumber, IOReader in, PrintWriter out) {
        int n = in.nextInt();
        int t = in.nextInt();
        String s1 = in.readLine();
        String s2 = in.readLine();
        char[] c1 = s1.toCharArray();
        char[] c2 = s2.toCharArray();
        char[] ans = new char[n];
        boolean ok = true;

        for (int i = 0; i < n; i++) {
            ans[i] = unique(c1[i], c2[i]);
        }

        int t1 = n;
        int t2 = n;
        if (n > t) {
            for (int i = 0; i < n; i++) {
                if (c1[i] == c2[i]) {
                    ans[i] = c1[i];
                    t1--;
                    t2--;
                }

                if (t1 == t && t2 == t) {
                    break;
                }
            }

            if (t1 > t || t2 > t) {
                for (int i = 0; i < n; i++) {
                    if (c1[i] != c2[i]) {
                        if (t1 > t2) {
                            ans[i] = c1[i];
                            t1--;
                        } else {
                            ans[i] = c2[i];
                            t2--;
                        }
                    }

                    if (t1 == t && t2 == t) {
                        break;
                    }
                }
            }
        }


        if (t1 == t && t2 == t) {
            out.println(String.valueOf(ans));
        } else {
            out.println(-1);
        }
    }

    char unique(char q1, char q2) {
        for (char l = 'a'; l <= 'z'; l++) {
            if (l != q1 && l != q2) {
                return l;
            }
        }
        return 'a';
    }

}

class IOReader {

    private BufferedReader reader;
    private StringTokenizer tokenizer;

    public String readLine() {
        try {
            return reader.readLine();
        } catch (IOException e) {
            e.printStackTrace();
            return null;
        }
    }

    public String nextToken() {
        while (tokenizer == null || !tokenizer.hasMoreTokens()) {
            try {
                tokenizer = new StringTokenizer(reader.readLine());
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return tokenizer.nextToken();
    }

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

    public IOReader(InputStream stream) {
        reader = new BufferedReader(new InputStreamReader(stream));
    }

}