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();
            String s1 = in.next();
            String s2 = in.next();

            if (T > N) {
                out.println(-1);
                return;
            }

            int different = diff(s1, s2);
            int equal = N - different;
            if (T >= different) {
                char ans[] = new char[N];

                int rem = T - different;
                for (int i = 0; i < N; i++) {
                    if (s1.charAt(i) != s2.charAt(i)) {
                        ans[i] = diffThan(s1.charAt(i), s2.charAt(i));
                    } else if (rem > 0) {
                        ans[i] = diffThan(s1.charAt(i), s2.charAt(i));
                        rem--;
                    } else {
                        ans[i] = s1.charAt(i);
                    }

                }
                out.println(new String(ans));
            } else {
                char ans[] = new char[N];

                int rem = 2 * (different - T);
                boolean first = true;
                for (int i = 0; i < N; i++) {
                    if (s1.charAt(i) != s2.charAt(i) && rem > 0) {
                        if (first) {
                            ans[i] = s1.charAt(i);
                        } else {
                            ans[i] = s2.charAt(i);
                        }
                        first = !first;
                        rem--;
                    } else if (s1.charAt(i) != s2.charAt(i)) {
                        ans[i] = diffThan(s1.charAt(i), s2.charAt(i));
                    } else {
                        ans[i] = s1.charAt(i);
                    }
                }
                if (rem > 0) {
                    out.println(-1);
                    return;
                }
                out.println(new String(ans));
            }
        }

        private char diffThan(char a, char b) {
            for (char c = 'a'; c <= 'z'; c++) {
                if (c != a && c != b) return c;
            }
            return 0;
        }

        private int diff(String s1, String s2) {
            int ans = 0;
            for (int i = 0; i < s1.length(); i++) {
                if (s1.charAt(i) != s2.charAt(i)) ans++;
            }
            return ans;
        }

    }

    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());
        }

    }
}