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.BufferedReader;
import java.io.InputStreamReader;
import java.math.BigInteger;

public class Main {
	public static void main(String[] args)throws Exception {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String[]in = br.readLine().split(" ");
		int n = Integer.parseInt(in[0]);
		int t = Integer.parseInt(in[1]);
		String s1 = br.readLine();
		String s2 = br.readLine();
		char[] s3 = new char[s1.length()];
		int diff =0;
		for(int i=0;i<n;i++){
			if(s1.charAt(i)!=s2.charAt(i))diff++;
		}
		if(t>=diff){
			int cnt=0;
			for(int i=0;i<n;i++){
				if(s1.charAt(i)!=s2.charAt(i)){
					for(int j=0;j<26;j++){
						if(j!=s1.charAt(i)-'a' && j!=s2.charAt(i)-'a'){
							s3[i]=(char)(j+'a');
							break;
						}
					}
					cnt++;
				}
			}
			for(int i=0;i<n;i++){
				if(s1.charAt(i)==s2.charAt(i) ){
					if(cnt<t){
						for(int j=0;j<26;j++){
							if(j!=s1.charAt(i)-'a'){
								s3[i]=(char)(j+'a');
								break;
							}
						}
						cnt++;
					}
					else
						s3[i]=s1.charAt(i);
				}
			}
			for(int i=0;i<n;i++)
				System.out.print(s3[i]);
			System.out.println();
		}
		else{
			int cnt1=0,cnt2=0,sim=0;
			for(int i=0;i<n;i++){
				if(s1.charAt(i)==s2.charAt(i)){
					s3[i]=s1.charAt(i);
					sim++;
					continue;
				}
				if(cnt1<t){
					s3[i]=s2.charAt(i);
					cnt1++;
				}
				else{
					s3[i]=s1.charAt(i);
				}
			}
			for(int i=0;i<n;i++){
				if(s3[i]!=s2.charAt(i)){
					cnt2++;
				}
			}
			for(int i=0;i<n;i++){
				if(s3[i]==s2.charAt(i) && s3[i]!=s1.charAt(i) && cnt2<t){
					cnt2++;
					for(int j=0;j<26;j++){
						if(j!=s1.charAt(i)-'a' && j!=s2.charAt(i)-'a'){
							s3[i]=(char)(j+'a');
							break;
						}
					}
				}
			}
			cnt1=cnt2=0;
			for(int i=0;i<n;i++){
				if(s3[i]!=s1.charAt(i))cnt1++;
				if(s3[i]!=s2.charAt(i))cnt2++;
			}
			if(!(cnt1==t && cnt2==t))
				System.out.println(-1);
			else{
				for(int i=0;i<n;i++)
					System.out.print(s3[i]);
				System.out.println();
			}
		}
		
	}
}