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 len = in.ni();
            int t = in.ni();
            String s1 = in.n();
            String s2 = in.n();
            StringBuilder sb = new StringBuilder(s1);
            boolean[] same = new boolean[len];
            t = len - t;
            int same_count = 0;
            if (t != 0) {
                for (int i = 0; i < len; ++i) {
                    if (s1.charAt(i) == s2.charAt(i)) {
                        same[i] = true;
                        --t;
                        ++same_count;
                    }
                    if (t == 0)
                        break;
                }
            }
            if (2 * t > len - same_count) {
                sb = new StringBuilder("-1");
            } else {
                for (int i = 0; i < len; ++i) {
                    if (t == 0) {
                        if (!same[i]) {
                            for (char c = 'a'; c <= 'z'; ++c) {
                                if (s1.charAt(i) != c && s2.charAt(i) != c) {
                                    sb.setCharAt(i, c);
                                    break;
                                }
                            }
                        }
                    } else if (!same[i]) {
                        sb.setCharAt(i, s2.charAt(i));
                        --t;
                        while (same[++i])
                            continue;
                    }
                }
            }
            out.println(sb.toString());
        }

    }

    static class InputReader {
        BufferedReader br;
        StringTokenizer st;

        public InputReader(InputStream inputStream) {
            br = new BufferedReader(new InputStreamReader(inputStream));
        }

        public String n() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    throw new RuntimeException();
                }
            }
            return st.nextToken();
        }

        public int ni() {
            return Integer.parseInt(n());
        }

    }
}