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

public class Rextester{
    static char finddif(char a,char b){
        for(char z = 'a';z<='z';z++){
            if(z!=a&&z!=b){
                return z;
            }
        }
        return 'w';
    }
    static char finddif(char a){
        for(char z='a';z<='z';z++){
            if(z!=a){
                return z;
            }
        }
        return 'w';
    }
    public static void main(String[] args)throws IOException{
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer st = new StringTokenizer(br.readLine());
        String a = br.readLine();
        String b = br.readLine();
        br.close();
        int sim=0,n = new Integer(st.nextToken()),t = new Integer(st.nextToken());
        for(int i=0;i<n;i++){
            if(a.charAt(i)==b.charAt(i)){
                sim++;
            }
        }
        char[] ans = new char[n];
        if(n-t<=sim){
            for(int i=0;i<n;i++){
                if(a.charAt(i)==b.charAt(i)){
                    if(t!=n){
                        ans[i]=a.charAt(i);
                        t++;
                    }
                    else{
                        ans[i]=finddif(a.charAt(i));
                    }
                }
                else{
                    ans[i]=finddif(a.charAt(i),b.charAt(i));
                }
            }
        }
        else{
            int ta = n-t-sim,tb = n-t-sim;
            //System.out.println(ta+" "+tb);
            for(int i=0;i<n;i++){
                if(a.charAt(i)==b.charAt(i)){
                    ans[i]=a.charAt(i);
                }
                else{
                    if(ta!=0){
                        ans[i]=a.charAt(i);
                        ta--;
                    }
                    else if(tb!=0){
                        ans[i]=b.charAt(i);
                        tb--;
                    }
                    else{
                        ans[i]=finddif(a.charAt(i),b.charAt(i));
                    }
                }
            }
            if(ta!=0||tb!=0){
                System.out.println("-1");
                return;
            }
        }
        StringBuffer sb = new StringBuffer();
        for(int i=0;i<n;i++){
            sb.append(ans[i]);
        }
        System.out.println(sb);
    }
}