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.IOException;
import java.io.OutputStream;
import java.io.PrintWriter;
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();
	}
}

class TaskC {
    public void solve(int testNumber, InputReader jin, PrintWriter jout) {
        int n = jin.int32();
        int t = jin.int32();

        String a = jin.token();
        String b = jin.token();

        int x = 0;
        for(int i = 0; i < n; i++) {
            if(a.charAt(i) != b.charAt(i))
                x++;
        }


        if(x > t) {
            int d = x - t;
            if(d > t) {
                jout.println(-1);
            } else {
                int d1 = d;
                int d2 = d;
                StringBuilder sb = new StringBuilder();
                for(int i = 0; i < n; i++) {
                    if (a.charAt(i) != b.charAt(i)) {
                        if(d1 > 0) {
                            sb.append(a.charAt(i));
                            d1--;
                        } else if(d2 > 0) {
                            sb.append(b.charAt(i));
                            d2--;
                        } else {
                            int aa = a.charAt(i) - 'a';
                            int bb = b.charAt(i) - 'a';
                            int c = (aa + 1) % 26;
                            if(c == bb) {
                                c = (c + 1) % 26;
                            }
                            char cc = (char)(c + 'a');
                            sb.append(cc);
                        }
                    } else {
                        sb.append(a.charAt(i));
                    }
                }
                jout.println(sb.toString());
            }
        } else {
            StringBuilder sb = new StringBuilder();
            int eq = t - x;
            for(int i = 0; i < n; i++) {
                if(a.charAt(i) != b.charAt(i)) {
                    int aa = a.charAt(i) - 'a';
                    int bb = b.charAt(i) - 'a';
                    int c = (aa + 1) % 26;
                    if(c == bb) {
                        c = (c + 1) % 26;
                    }
                    char cc = (char)(c + 'a');
                    sb.append(cc);
                } else if(eq > 0) {
                    int bb = b.charAt(i) - 'a';
                    int c = (bb + 1) % 26;
                    char cc = (char)(c + 'a');
                    sb.append(cc);
                    eq--;
                } else {
                    sb.append(a.charAt(i));
                }
            }
            jout.println(sb.toString());
        }
    }
}

class InputReader {
    private static final int bufferMaxLength = 1024;
    private InputStream in;
    private byte[] buffer;
    private int currentBufferSize;
    private int currentBufferTop;
    private static final String tokenizers = " \t\r\f\n";

    public InputReader(InputStream stream) {
        this.in = stream;
        buffer = new byte[bufferMaxLength];
        currentBufferSize = 0;
        currentBufferTop = 0;
    }

    private boolean refill() {
        try {
            this.currentBufferSize = this.in.read(this.buffer);
            this.currentBufferTop = 0;
        } catch(Exception e) {}
        return this.currentBufferSize > 0;
    }


    public Byte readChar() {
        if(currentBufferTop < currentBufferSize) {
            return this.buffer[this.currentBufferTop++];
        } else {
            if(!this.refill()) {
                return null;
            } else {
                return readChar();
            }
        }
    }

    public String token()  {
        StringBuffer tok = new StringBuffer();
        Byte first;
        while((first = readChar()) != null && (tokenizers.indexOf((char) first.byteValue()) != -1));
        if(first == null) return null;
        tok.append((char)first.byteValue());
        while((first = readChar()) != null && (tokenizers.indexOf((char) first.byteValue()) == -1)) {
            tok.append((char)first.byteValue());
        }
        return tok.toString();
    }


    public String next() {
        return token();
    }

    public Integer int32() throws NumberFormatException {
        String tok = token();
        return tok == null? null : Integer.parseInt(tok);
    }

    public Long int64() throws NumberFormatException {
        String tok = token();
        return tok == null? null : Long.parseLong(tok);
    }
}