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.BufferedReader;
import java.io.InputStreamReader;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author ale64bit
 */
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.next().toCharArray();
            char[] s2 = in.next().toCharArray();

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

            boolean odd = (d & 1) != 0;
            char[] s3 = new char[n];
            for (int i = 0, k = 0; i < n; ++i) {
                if (s1[i] != s2[i]) {
                    if (odd) {
                        odd = false;
                        char c = 'a';
                        while (s1[i] == c || s2[i] == c) ++c;
                        s3[i] = c;
                    } else {
                        s3[i] = k == 0 ? s1[i] : s2[i];
                        k = 1 - k;
                    }
                } else {
                    s3[i] = s1[i];
                }
            }

//        out.println("After pairing: " + String.valueOf(s3));
            int cur1 = 0, cur2 = 0;
            for (int i = 0; i < n; ++i) {
                if (s1[i] != s3[i]) {
                    ++cur1;
                }
                if (s2[i] != s3[i]) {
                    ++cur2;
                }
            }

//        out.println("  cur1 diff " + cur1);
//        out.println("  cur2 diff " + cur2);
            for (int i = 0; i < n && (cur1 < t || cur2 < t); ++i) {
                if (s1[i] == s2[i]) {
                    char c = 'a';
                    while (s1[i] == c || s2[i] == c) ++c;
                    s3[i] = c;
                    ++cur1;
                    ++cur2;
                }
            }

            for (int i = 0; i < n && (cur1 < t || cur2 < t); ++i) {
                if (s1[i] == s3[i] || s2[i] == s3[i]) {
                    char c = 'a';
                    while (s1[i] == c || s2[i] == c) ++c;
                    if (s1[i] == s3[i]) {
                        ++cur1;
                    } else {
                        ++cur2;
                    }
                    s3[i] = c;
                }
            }

            if (cur1 == t && cur2 == t) {
                out.println(String.valueOf(s3));
            } else {
                out.println(-1);
            }
        }

    }

    static class FastScanner {
        BufferedReader br;
        StringTokenizer st;

        public FastScanner(InputStream inputStream) {
            br = new BufferedReader(new InputStreamReader(inputStream));
            st = new StringTokenizer("");
        }

        public String next() {
            while (!st.hasMoreTokens()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (Exception e) {
                    return null;
                }
            }
            return st.nextToken();
        }

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

    }
}