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.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.Scanner;
import java.util.concurrent.ArrayBlockingQueue;
 
public class Main {
    Scanner sc = new Scanner(System.in);
 
    public static void main(String[] args) {
        new Main(); 
    }
 
    public Main() {
        new C().doIt();
    }
    class C{
        char chara(char c1,char c2){
            for(char ctr = 'a';ctr <= 'z';ctr++){
                if(c1 != ctr && c2 != ctr){
                    return ctr;
                }
            }
            return 'z';
        }
        
        char schara(char c1){
            for(char ctr = 'a';ctr <= 'z';ctr++){
                if(c1 == ctr){
                    return ctr;
                }
            }
            return 'z';
        }
        void overmatch(char c1[],char c2[],int t2,int n){
            StringBuilder sb = new StringBuilder("");
            for(int i = 0;i < n;i++){
                if(t2 > 0){
                    if(c1[i] == c2[i]){
                        sb.append(c1[i]);
                        t2--;
                    }else sb.append(chara(c1[i],c2[i]));
                }else sb.append(chara(c1[i],c2[i]));
            }
            System.out.println(sb);
        }
        
        void lowmatch(char c1[],char c2[],int t2,int n,int m){
            int s1 = m,s2 = m;
            StringBuilder sb = new StringBuilder("");
            for(int i = 0;i < n;i++){
                if(c1[i] == c2[i])sb.append(c1[i]);
                else if(s1 > 0){
                    s1--;
                    sb.append(schara(c1[i]));
                }
                else if(s2 > 0){
                    s2--;
                    sb.append(schara(c2[i]));
                }
                else sb.append(chara(c1[i],c2[i]));
            }
            System.out.println(sb);
        }
        void doIt(){
            int n = sc.nextInt();
            int t = sc.nextInt();
            String s1 = sc.next();
            String s2 = sc.next();
            char c1[] = s1.toCharArray();
            char c2[] = s2.toCharArray();
            int match = 0,t2 = n - t;
            for(int i = 0;i < n;i++){
                if(c1[i] == c2[i])match++;
            }
            int nmatch = n - match;
            if(nmatch < (t2 - match)*2)System.out.println(-1);
            else{
                if(t2 <= match)overmatch(c1,c2,t2,n);
                else lowmatch(c1,c2,t2,n,t2 - match);
            }
//          System.out.println((t2 - match)+" "+match+" "+t2);
        }
    }
}