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.io.*;
public class Main {
public static void main(String [] args) throws Exception{
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer(br.readLine()); 
    int n = Integer.parseInt(st.nextToken());
    int t = Integer.parseInt(st.nextToken());
    String str1 = br.readLine();
    String str2 = br.readLine();
    int pos = 0;
    StringBuilder sb = new StringBuilder();
            for(int i = 0;i < n;i++){
            if(str1.charAt(i) != str2.charAt(i))pos++;
}
   //System.out.println((pos <= t) + " " + pos);
   if(pos <= t){
   int change = t - pos;
   //System.out.println(change);
   for(int i = 0;i < n;i++){
   if(str1.charAt(i) == str2.charAt(i) && change > 0){
   int num = str1.charAt(i) - 95;
   if(num == 27)num = 1;
          sb.append((char)(num + 96));
          --change;
}
   else if(str1.charAt(i) != str2.charAt(i)){
   int num1 = str1.charAt(i) - 96;
   int num2 = str2.charAt(i) - 96;
   int num = 1;
   while(num == num1 || num2 == num)++num;
   //System.out.println((char)(num + 48));
   sb.append((char)(num + 96));
}
else sb.append(str1.charAt(i));
}
System.out.print(sb);
return;
}

    if(pos > 2*t){
    System.out.print("-1");
    return;
}
int temp = pos - t;
int diff = t - temp;
int ptr= 0; 
//System.out.println(sb);   
         for(int i = 0;i < str1.length();i++){
         if(str1.charAt(i) != str2.charAt(i) && diff > 0){
         int num1 = str1.charAt(i) - 96;
         int num2 = str2.charAt(i) - 96;
         int num = 1;
         while(num == num1 || num == num2)++num;
          sb.append((char)(num + 96));
          --diff;
        }
         else if(str2.charAt(i) == str1.charAt(i))
         sb.append(str1.charAt(i));
         
         else if(str1.charAt(i) != str2.charAt(i) && ptr < temp){
         sb.append(str1.charAt(i));++ptr;
}
         
         else if(str1.charAt(i) != str2.charAt(i) && ptr < 2*temp){
         sb.append(str2.charAt(i));++ptr;
}
         //System.out.println(sb);
}
System.out.print(sb);
}
}