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.BufferedReader;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 * @author Tifuera
 */
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();
	}
}

class TaskC {
    public void solve(int testNumber, InputReader in, PrintWriter out) {
        int n = in.nextInt();
        int t = in.nextInt();

        String first = in.next();
        String second = in.next();

        int distinct = getDistinct(first, second);
        int same = first.length() - distinct;

        int diffWithSingleInDistinct = 0;
        int diffInSameCandidate = 0;
        while (diffWithSingleInDistinct <= distinct) {
            int diffWithBoth = distinct - diffWithSingleInDistinct;
            diffInSameCandidate = (2 * t - 2 * diffWithBoth - diffWithSingleInDistinct) / 2;
            if (diffInSameCandidate >= 0 && diffInSameCandidate <= same) {
                break;
            }

            diffWithSingleInDistinct += 2;
        }

        if (diffWithSingleInDistinct > distinct) {
            out.println(-1);
            return;
        }

        for (int i = 0; i < n; i++) {
            if (first.charAt(i) == second.charAt(i)) {
                if (diffInSameCandidate > 0) {
                    out.print(getDifferentChar(first.charAt(i), second.charAt(i)));
                    diffInSameCandidate--;
                } else {
                    out.print(first.charAt(i));
                }
            } else {
                if (diffWithSingleInDistinct > 0) {
                    if (diffWithSingleInDistinct % 2 == 1) {
                        out.print(first.charAt(i));
                    } else {
                        out.print(second.charAt(i));
                    }
                    diffWithSingleInDistinct--;
                } else {
                    out.print(getDifferentChar(first.charAt(i), second.charAt(i)));
                }
            }
        }
    }

    private char getDifferentChar(char first, char second) {
        char ch = 'a';
        while (ch == first || ch == second) {
            ch++;
        }
        return ch;
    }

    private int getDistinct(String first, String second) {
        int res = 0;
        for (int i = 0; i < first.length(); i++) {
            if (first.charAt(i) != second.charAt(i)) {
                res++;
            }
        }

        return res;
    }

}

class InputReader {
    private BufferedReader reader;
    private String[] currentArray;
    private int curPointer;

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


    public String next() {
        try {
            currentArray = null;
            return reader.readLine();
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    public int nextInt() {
        if ((currentArray == null) || (curPointer >= currentArray.length)) {
            try {
                currentArray = reader.readLine().split(" ");
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
            curPointer = 0;
        }
        return Integer.parseInt(currentArray[curPointer++]);
    }

}