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.Arrays;
import java.util.PriorityQueue;
import java.util.Scanner;
import java.util.Vector;




public class b {

    
	//static int[]visit;
	static int[]m;
	//static int[][]temp;
	public static void main(String args[]) {
		          
            
            Scanner sc=new Scanner(System.in);
         
            int n=sc.nextInt();
            int t=sc.nextInt();
            
            char[] s1=sc.next().toCharArray();
            char[] s2=sc.next().toCharArray();
            int[]a1=new int[26];
            int[]a2=new int[26];
         
           int h=n-t;
            int n1=h,n2=h;
       
           
           char[]c=new char[n];
           
           
           for(int i=0;i<n;i++)
           {
               if(n1==0&&n2==0)break;
               if(s1[i]==s2[i]&&n1>0&&n2>0)
               {
                   n1--;
                   n2--;
                   c[i]=s1[i];
               }
              
             
               
               
           }
           for(int i=0;i<n;i++)
           {
               if(n1==0&&n2==0)break;
              
               if(s1[i]!=s2[i]&&n1>0)
               {
                   n1--;
                   
                   c[i]=s1[i];
               }
               else
               if(s1[i]!=s2[i]&&n2>0)
               {
                   n2--;
                   
                   c[i]=s2[i];
               }
             
               
               
           }
           
           if(n1>0||n2>0)
                System.out.println("-1");
           else
           {
               
               for (int i = 0; i < n; i++) {
                   if(c[i]<'a'||c[i]>'z'){
                   for (int j = 0; j < 26; j++) {
                       char as=(char)(j+'a');
                       if(s1[i]!=as&&s2[i]!=as)
                       {c[i]=as;
                       break;
                       }
                   }
               }
               }
               System.out.println(String.valueOf(c));
           }
            
          
            
            
}

        
        
}