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.*;
public class ProC {
	static StreamTokenizer in=new StreamTokenizer(new BufferedReader(new InputStreamReader(System.in)));
	static int nextInt() throws IOException    
	{  
	    in.nextToken();    
	    return (int)in.nval;     
	}
	static String next() throws IOException    
	{  
	    in.nextToken();    
	    return in.sval;     
	}
	static PrintWriter out=new PrintWriter(new OutputStreamWriter(System.out));
	static int n,t,ct,cc;
	static String aa,bb;
	static char d,a[],b[],c[]=new char[200000];
	static int[] mm=new int[200000];
	static int[] m=new int[200000];

	public static void main(String[] args) throws IOException {
		n=nextInt();t=n-nextInt();
		aa=next();bb=next();
		a=aa.toCharArray();b=bb.toCharArray();
		for(int i=0;i<n;i++)
			if(a[i]==b[i]) { mm[i]=1;ct++;}
		if(t>ct+(n-ct)/2) out.println(-1);
		else
		{
			for(int i=0;i<n;i++)
			{
				if(cc==t) break;
				if(mm[i]==1)
				{
					c[i]=a[i];
					m[i]=1;cc++;
				}	
			}
			if(cc<t)
			{
				cc=t-cc;
				ct=0;
				for(int i=0;i<n;i++)
				{
					if(m[i]==0) { c[i]=a[i];ct++;m[i]=1;}
					if(ct==cc) break;
				}
				ct=0;
				for(int i=0;i<n;i++)
				{
					if(m[i]==0) { c[i]=b[i];ct++;m[i]=1;}
					if(ct==cc) break;
				}
			}
			for(int i=0;i<n;i++)
			{
				if(m[i]==0)
				{
					for(int j=0;j<26;j++)
					{
						d=(char)('a'+j);
						if(d!=a[i]&&d!=b[i])
						{
							c[i]=d;break;
						}
					}
				}
			}
			for(int i=0;i<n;i++)
				out.print(c[i]);
			out.println();
		}
		out.flush();
	}
}