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.HashSet;
import java.util.Scanner;
import java.util.Set;

/**
 * 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;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        c584 solver = new c584();
        solver.solve(1, in, out);
        out.close();
    }

    static class c584 {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            int n = in.nextInt();
            int t = in.nextInt();
            String s1 = in.next();
            String s2 = in.next();
            Set<Integer> same = new HashSet<>();
            for (int i = 0; i < n; i++) {
                if (s1.charAt(i) == s2.charAt(i)) {
                    if (same.size() == n - t) break;
                    same.add(i);
                }
            }
            if (same.size() < n - t) {
                char[] ans = new char[n];
                for (int i = 0; i < n; i++) {
                    if (same.contains(i)) {
                        ans[i] = s1.charAt(i);
                    }
                }
                int same1 = same.size();
                int same2 = same.size();
                for (int i = 0; i < n; i++) {
                    if (!same.contains(i)) {
                        if (same1 > same2) {
                            ans[i] = s2.charAt(i);
                        } else if (same1 < n - t) {
                            ans[i] = s1.charAt(i);
                        } else {
                            for (char c = 'a'; c <= 'z'; c++) {
                                if (s2.charAt(i) != c && s1.charAt(i) != c) {
                                    ans[i] = c;
                                    break;
                                }
                            }
                        }
                        if (ans[i] == s2.charAt(i)) same2++;
                        if (ans[i] == s1.charAt(i)) same1++;
                    }
                }
                if (same1 != n - t || same2 != n - t) {
                    out.print(-1);
                } else {
                    for (char c : ans) out.print(c);
                }
            } else {
                for (int i = 0; i < n; i++) {
                    if (same.contains(i)) {
                        out.print(s1.charAt(i));
                    } else {
                        for (char j = 'a'; j <= 'z'; j++) {
                            if (s1.charAt(i) != j && s2.charAt(i) != j) {
                                out.print(j);
                                break;
                            }
                        }
                    }
                }
            }
        }

    }
}