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
 *
 * @author Artyom Korzun
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        FastScanner in = new FastScanner(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, FastScanner in, PrintWriter out) {
            int n = in.nextInt();
            int t = in.nextInt();
            char[] s1 = in.nextWord();
            char[] s2 = in.nextWord();

            int diffs = 0;
            for (int i = 0; i < n; i++) {
                if (s1[i] != s2[i])
                    diffs++;
            }

            int sim = diffs - t;
            if (sim > t) {
                out.print(-1);
                return;
            } else if (sim >= 0) {
                int i = 0;
                for (int simCount = 0; simCount < sim; i++)
                    if (s1[i] != s2[i])
                        simCount++;

                for (int simCount = 0; simCount < sim; i++) {
                    if (s1[i] != s2[i]) {
                        s1[i] = s2[i];
                        simCount++;
                    }
                }

                for (; i < n; i++)
                    if (s1[i] != s2[i])
                        s1[i] = next(s1[i], s2[i]);
            } else {
                for (int i = 0; i < n; i++) {
                    if (s1[i] == s2[i]) {
                        if (sim < 0) {
                            s1[i] = next(s1[i], s2[i]);
                            sim++;
                        }
                    } else {
                        s1[i] = next(s1[i], s2[i]);
                    }
                }
            }

            out.print(s1);

        }

        static char next(char a, char b) {
            char c = 'a';
            while (c == a || c == b)
                c++;

            return c;
        }

    }

    static class FastScanner {
        private final BufferedReader reader;
        private StringTokenizer tokenizer;

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

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

        public char[] nextWord() {
            return next().toCharArray();
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens())
                tokenizer = new StringTokenizer(nextLine());

            return tokenizer.nextToken();
        }

        public String nextLine() {
            try {
                return reader.readLine();
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }

    }
}