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.util.*;
import java.lang.*;
import java.io.*;

// Created by @thesupremeone on 22/03/21
public class MarinaAndVasya {
    BufferedReader in;
    BufferedWriter out;
    StringTokenizer st;
    boolean onlineJudge;

    char getDiffChar(char a, char b){
        char c = 'a';
        while (c==a || c==b)
            c++;
        return c;
    }
    
    // My Solution
    void solve() throws IOException {
        int n = getInt();
        int t = getInt();
        String s1 = getLine();
        String s2 = getLine();
        int diff = 0;
        for (int i = 0; i < n; i++) {
            if(s1.charAt(i)!=s2.charAt(i))
                diff++;
        }
        if(t==0 && diff==0){
            println(s1);
        }else if(t<Math.ceil(diff/2.0) || t>n){
            println("-1");
        }else {
            int updateSame = t>diff ? t-diff : 0;
            int sameAsS1 = diff>t ? diff-t : 0;
            int sameAsS2 = sameAsS1;
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < n; i++) {
                char a = s1.charAt(i);
                char b = s2.charAt(i);
                if(a==b){
                    if(updateSame>0){
                        sb.append(getDiffChar(a, b));
                        updateSame--;
                    }else {
                        sb.append(a);
                    }
                }else {
                    if(sameAsS1>0){
                        sb.append(a);
                        sameAsS1--;
                    }else if(sameAsS2>0){
                        sb.append(b);
                        sameAsS2--;
                    }else {
                        sb.append(getDiffChar(a, b));
                    }
                }
            }
            println(sb.toString());
        }
    }
    // Handling CodeExecution
    public static void main(String[] args) throws Exception {
        new MarinaAndVasya().run();
    }
    void run() throws IOException {
        onlineJudge = isOnlineJudge();
        // Defining Input Streams
        if (onlineJudge) {
            in = new BufferedReader(new InputStreamReader(System.in));
            out = new BufferedWriter(new OutputStreamWriter(System.out));
        } else {
            in = new BufferedReader(new FileReader("input.txt"));
            out = new BufferedWriter(new FileWriter("output.txt"));
        }
        // Running Logic
        solve();
        out.flush();
        // Run example test cases
        if (!onlineJudge) {
            BufferedReader output = new BufferedReader(new FileReader("output.txt"));
            BufferedReader answer = new BufferedReader(new FileReader("answer.txt"));
            StringBuilder outFile = new StringBuilder();
            StringBuilder ansFile = new StringBuilder();
            String temp;
            while ((temp = output.readLine()) != null)
                outFile.append(temp.trim());
            while ((temp = answer.readLine()) != null)
                ansFile.append(temp.trim());
            if (outFile.toString().equals(ansFile.toString()))
                System.out.println("Test Cases Passed!!!");
            else
                System.out.println("Failed...");
        }
    }
    // Checks whether the code is running on OnlineJudge or LocalSystem
    boolean isOnlineJudge() {
        try {
            return System.getProperty("ONLINE_JUDGE")!=null
                    || System.getProperty("LOCAL")==null;
        } catch (Exception e) {
            return true;
        }
    }
    // Some functions
    void print(String... s) throws IOException {
        for (String value : s) out.write(value);
    }
    void println(String... s) throws IOException{
        print(s);
        out.newLine();
    }
    String getLine() throws IOException{
        return in.readLine();
    }
    String getToken() throws IOException{
        if(st==null || !st.hasMoreTokens())
            st = new StringTokenizer(getLine());
        return st.nextToken();
    }
    int getInt() throws IOException {
        return Integer.parseInt(getToken());
    }
    long getLong() throws IOException {
        return Long.parseLong(getToken());
    }
    int[] getInts(int n) throws IOException {
        int[] a = new int[n];
        for (int i = 0; i < n; i++)
            a[i] = getInt();
        return a;
    }
    long[] getLongs(int n) throws IOException {
        long[] a = new long[n];
        for (int i = 0; i < n; i++)
            a[i] = getLong();
        return a;
    }
    int[][] getIntMat(int n, int m) throws IOException {
        int[][] mat = new int[n][m];
        for (int i = 0; i < n; i++)
            for (int j = 0; j < m; j++)
                mat[i][j] = getInt();
        return mat;
    }
    char[][] getCharMat(int n, int m) throws IOException {
        char[][] mat = new char[n][m];
        for (int i = 0; i < n; i++) {
            String s = getLine();
            for (int j = 0; j < m; j++)
                mat[i][j] = s.charAt(j);
        }
        return mat;
    }
}