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

/**
 * 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;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        TaskC solver = new TaskC();
        solver.solve(1, in, out);
        out.close();
    }

    static class TaskC {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int n = in.nextInt();
            int t = in.nextInt();
            char[] a = in.next().toCharArray();
            char[] b = in.next().toCharArray();
            int dif = 0;
            for (int i = 0; i < n; i++) {
                if (a[i] != b[i]) ++dif;
            }
            char[] c = new char[n];
            if (t < dif) {
                int count = 0;
                int count2 = 0;
                int need = dif - t;
                for (int i = 0; i < n; i++) {
                    if (a[i] != b[i]) {
                        if (count < need) {
                            c[i] = a[i];
                            ++count;
                        } else if (count2 < need) {
                            c[i] = b[i];
                            ++count2;
                        } else {
                            char cur = 'a';
                            while (cur == a[i] || cur == b[i]) {
                                ++cur;
                            }
                            c[i] = cur;
                        }
                    } else {
                        c[i] = a[i];
                    }
                }
                int recal1 = 0;
                int recal2 = 0;
                for (int i = 0; i < n; i++) {
                    if (a[i] != c[i]) ++recal1;
                    if (b[i] != c[i]) ++recal2;
                }
                if (recal1 != t || recal2 != t) {
                    out.println(-1);
                    return;
                }
                out.println(new String(c));
                return;
            }
            int count = t - dif;
            for (int i = 0; i < n; i++) {
                if (a[i] == b[i]) {
                    if (count > 0) {
                        char cur = 'a';
                        while (cur == a[i]) {
                            cur++;
                        }
                        c[i] = cur;
                        --count;
                    } else {
                        c[i] = a[i];
                    }
                } else {
                    char cur = 'a';
                    while (cur == a[i] || cur == b[i]) {
                        ++cur;
                    }
                    c[i] = cur;
                }
            }
            out.println(new String(c));
        }

    }

    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

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

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

    }
}