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.OutputStream;
import java.io.PrintWriter;
import java.io.BufferedWriter;
import java.util.InputMismatchException;
import java.io.IOException;
import java.io.FileInputStream;
import java.util.ArrayList;
import java.io.Writer;
import java.io.OutputStreamWriter;
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);
        OutputWriter out = new OutputWriter(outputStream);
        CMarinaAndVasya solver = new CMarinaAndVasya();
        solver.solve(1, in, out);
        out.close();
    }

    static class CMarinaAndVasya {
        public void solve(int testNumber, InputReader in, OutputWriter out) {
            int n = in.nextInt(), t = in.nextInt();
            String str1 = in.nextLine(), str2 = in.nextLine();
            int P = -1, Q = -1, R = -1, S = -1, k = 0, m = 0;
            for (int i = 0; i < n; i++) {
                if (str1.charAt(i) == str2.charAt(i)) m++;
                else k++;
            }
            for (int p = 0; p <= k; p++) {
                int r = k - 2 * p;
                if (!(r >= 0 && r <= k)) continue;
                int s = t - (p + r);
                if (!(s >= 0 && s <= m)) continue;
                P = p;
                Q = p;
                R = r;
                S = s;
                break;
            }
            if (P == -1) {
                out.println(-1);
                out.flush();
                return;
            }
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < n; i++) {
                if (str1.charAt(i) != str2.charAt(i)) {
                    if (P > 0) {
                        sb.append(str1.charAt(i));
                        P--;
                    } else if (Q > 0) {
                        sb.append(str2.charAt(i));
                        Q--;
                    } else if (R > 0) {
                        char ch = 'a';
                        if (str1.charAt(i) == ch || str2.charAt(i) == ch) ch++;
                        if (str1.charAt(i) == ch || str2.charAt(i) == ch) ch++;
                        sb.append(ch);
                        R--;
                    }
                } else {
                    if (S > 0) {
                        char ch = 'a';
                        if (str1.charAt(i) == ch) ch++;
                        sb.append(ch);
                        S--;
//                    out.println("HHHH");
                    } else sb.append(str1.charAt(i));
                }
            }
            out.println(sb);
            out.flush();
        }

    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[8192];
        private int curChar;
        private int numChars;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        public InputReader(FileInputStream file) {
            this.stream = file;
        }

        public int read() {
            if (numChars == -1) {
                throw new InputMismatchException();
            }
            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (numChars <= 0) {
                    return -1;
                }
            }
            return buf[curChar++];
        }

        public int nextInt() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            int res = 0;
            do {
                if (c < '0' || c > '9') {
                    throw new InputMismatchException();
                }
                res = (res << 3) + (res << 1) + c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public String nextLine() {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            StringBuilder res = new StringBuilder();
            do {
                res.appendCodePoint(c);
                c = read();
            } while (!isEndOfLine(c));
            return res.toString();
        }

        private boolean isSpaceChar(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        private boolean isEndOfLine(int c) {
            return c == '\n' || c == '\r' || c == -1;
        }

    }

    static class OutputWriter {
        private final PrintWriter writer;
        private ArrayList<String> res = new ArrayList<>();
        private StringBuilder sb = new StringBuilder("");

        public OutputWriter(OutputStream outputStream) {
            writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
        }

        public OutputWriter(Writer writer) {
            this.writer = new PrintWriter(writer);
        }

        public void println(Object... objects) {
            for (int i = 0; i < objects.length; i++) {
                sb.append(objects[i]);
            }
            res.add(sb.toString());
            sb = new StringBuilder("");
        }

        public void close() {
            writer.close();
        }

        public void flush() {
            for (String str : res) writer.printf("%s\n", str);
            res.clear();
            sb = new StringBuilder("");
        }

    }
}